Hey there, WordPress user! Let‘s dive deep into the concept of thumbnail sizes in WordPress. Understanding how thumbnails work is crucial for optimizing your website‘s media performance and storage usage.
In this comprehensive guide, we‘ll cover everything from the basics of what thumbnail sizes are to advanced techniques for streamlining your media library. Whether you‘re a beginner just learning the ropes or a seasoned developer looking to fine-tune your setup, you‘ll find valuable insights and actionable tips below.
What Are Thumbnail Sizes in WordPress?
Thumbnails in WordPress refer to the various resized versions of an image that are automatically generated when you upload media to your site. But here‘s the key thing to understand: "thumbnail" is a bit of a misnomer. These resized images are not just small thumbnails!
WordPress generates multiple copies of your uploaded image in different dimensions. By default, WordPress creates three sizes in addition to the original full-size image:
| Name | Dimensions |
|---|---|
| Thumbnail | 150 x 150 px |
| Medium | 300 x 300 px |
| Large | 1024 x 1024 px |
So when you upload a photo from your camera, WordPress will create resized versions at those specific dimensions. But here‘s where the real magic happens: themes and plugins can define their own custom image sizes too!
For example, let‘s say you‘re running an e-commerce store using WooCommerce. The plugin might specify a "product catalog" thumbnail size of 300×300 and a "single product" size of 600×600. When you upload a product photo, poof, WordPress automatically generates those additional sizes.
The whole point of this system is to allow your theme to use optimally sized images across your site‘s front-end. Instead of loading a massive 5000px wide original image, your theme can instead output the perfectly resized 600px "single product" thumbnail.
Serving appropriately sized images is crucial for your site‘s performance. According to HTTP Archive, images make up on average 50% of a web page‘s weight. So having an optimized thumbnail system can make a huge impact on load times!
How WordPress Generates Thumbnail Sizes
When you upload an image in the WordPress media library, a lot happens behind the scenes to generate those thumbnail sizes. Let‘s geek out for a moment and look at the technical details.
The heavy lifting happens in the wp_generate_attachment_metadata function in /wp-includes/media.php. This function:
- Checks the image type (JPEG, PNG, WebP, etc.)
- Determines the original image dimensions
- Loops through the defined thumbnail sizes
- Calls
image_resizeto create each resized version - Saves the resized images in the
/uploadsdirectory - Returns an array of metadata about the generated sizes
For theme and plugin developers, the key step is registering your custom thumbnail sizes using add_image_size. For example, let‘s say we want to add a "Portfolio Thumbnail" of 400px x 250px cropped. In the functions.php file, we‘d add:
add_image_size( ‘portfolio-thumb‘, 400, 250, true );The true parameter tells WordPress to hard crop the image to those exact dimensions. If we set it to false, WordPress would scale the image proportionally to fit within a 400×250 box.
The Problem with Too Many Thumbnail Sizes
Now, if you‘re thinking "the more thumbnail sizes, the better!", hang on a moment. There‘s a big catch to generating lots of image sizes: storage space!
Each resized thumbnail image takes up space on your web server. A theme or plugin that defines a ton of sizes can massively bloat your media library.
In an analysis of over 4 million WordPress websites, the team at WPMU DEV found some shocking statistics:
- The average WordPress site has 162 images
- 10% of WordPress sites have 500+ images
- The average total filesize of images on a WordPress site is 252 MB
- Having 5 extra thumbnail sizes (beyond the WordPress defaults) increases storage used per uploaded image by 252 KB on average
Yikes! As you can imagine, having too many thumbnails can really add up, especially on media-heavy sites. Throw in ultra high-resolution photos from modern cameras and you can easily eat up multiple gigabytes of storage.
So what‘s a website owner to do? The key is finding the right balance of thumbnail sizes for your specific site‘s needs. In general, most sites really only need 4-5 sizes total.
Before defining a custom size, ask yourself:
- Is this size absolutely necessary for the theme design?
- Could an existing size work instead with some CSS adjustments?
- Is this size likely to be re-used across the site? Or is it a one-off?
If you‘re a theme or plugin developer, carefully consider what sizes to include, and please provide an easy way for users to disable sizes they don‘t need! Your users‘ server space will thank you.
Regenerating Thumbnail Sizes
Now, let‘s say you‘ve realized you have way too many thumbnail sizes on your site. Or maybe you‘ve switched to a new theme that requires different sizes than what you‘ve previously defined. What do you do with all those old images?
By default, when you register a new thumbnail size (or change the dimensions of an existing one), WordPress will only generate that new size for new image uploads. It won‘t go back and retroactively create the new size for images already in your media library.
That‘s where thumbnail regeneration plugins come in! These handy tools allow you to recreate thumbnail sizes for all images that have already been uploaded.
Some of the most popular regenerate thumbnail plugins include:
- Regenerate Thumbnails by Alex Mills
- Force Regenerate Thumbnails by Pedro Elsner
- AJAX Thumbnail Rebuild by junkcoder
Using one of these plugins is quite simple: install, activate, then click the "Regenerate Thumbnails" button. The plugin will work its way through your media library and generate the missing thumbnail sizes.
Fair warning though: regenerating thumbnails on a large site can take some time! It‘s a resource-intensive process, so it‘s best to run it during low-traffic periods.
Also, make sure you have a backup of your media library before regenerating, just in case anything goes wrong. Better safe than sorry!
Advanced Techniques for Optimizing Thumbnails
Alright, time to level up! Let‘s look at a few advanced tips and techniques for really fine-tuning your thumbnail game.
Lazy Loading
Lazy loading is a technique where images are only loaded when they‘re visible on the user‘s screen. This can have a big impact on perceived performance, especially for long pages with many images.
There are a few ways to implement lazy loading in WordPress:
- Use a lazy loading plugin like BJ Lazy Load or a3 Lazy Load
- Use the native lazy loading feature added in WordPress 5.5 (just add
loading="lazy"to your<img>tags) - Implement lazy loading in your theme template files manually with JavaScript
With native browser-level lazy loading support becoming more widespread, I‘d recommend using the HTML loading attribute whenever possible. It‘s a dead-simple one-liner to add to your image tags.
WebP Images
WebP is a modern image format developed by Google that provides significantly better compression than JPEG or PNG. We‘re talking like 25-35% smaller file sizes without sacrificing quality!
WordPress actually has built-in support for WebP since version 5.8. If you upload a .webp image to the media library, WordPress will use it in the front-end of your site where supported.
The catch is, not all browsers fully support WebP yet, so you‘ll need to provide a fallback. That‘s where plugins like WebP Express or Imagify come in handy. They‘ll automatically serve WebP images to supported browsers while gracefully falling back to JPEG/PNG for older browsers.
Making the switch to WebP for your thumbnails and other images can shave off a sizeable chunk of page weight, so it‘s well worth considering!
Image Compression
In addition to using an optimized image format like WebP, compressing your thumbnail images can also drastically reduce file sizes. Compression reduces the amount of data used to represent the image, at the cost of some quality loss.
There are two main types of image compression:
- Lossy – removes data from the image permanently; smaller file sizes but noticeable quality loss at high compression levels (e.g. JPEG)
- Lossless – reduces file size without removing data; file sizes aren‘t as small as lossy but quality is preserved (e.g. PNG)
In general, lossy compression is best for photographs, while lossless is better for graphics with sharp lines and text. For most thumbnail images, lossy JPEG compression at a quality level of 70-80% is a good balance.
You can use image optimization plugins like ShortPixel, Smush, or Imagify to automatically compress your media library. Some of these can even compress images as they‘re uploaded!
Just be careful not to overdo it with compression. Overly aggressive compression can lead to unsightly artifacts and quality issues, especially on larger featured images. When in doubt, err on the side of lighter compression.
Frequently Asked Questions
Before we wrap up, let‘s address some of the most common questions and issues that come up with WordPress thumbnail sizes.
How do I change the default WordPress thumbnail sizes?
To change the default thumbnail dimensions, head to Settings → Media in your WordPress admin dashboard. There you can specify the max width and height for thumbnail, medium, and large image sizes.
Keep in mind these settings will only apply to new image uploads, not existing images. See the section on "Regenerating Thumbnail Sizes" above for dealing with old images.
How can I disable thumbnail generation entirely?
If for some reason you want to completely disable WordPress‘ thumbnail generation, you can add this snippet to your theme‘s functions.php file or a site-specific plugin:
add_filter( ‘intermediate_image_sizes‘, ‘__return_empty_array‘ );This will prevent WordPress from generating any resized image versions when you upload new media. Be careful with this – many themes and plugins rely on thumbnail sizes and will break if they‘re disabled entirely!
Why are my thumbnail images blurry or low-quality?
Blurry or low-quality thumbnails are usually caused by overly aggressive compression. When you upload an image, WordPress first creates a copy of the original, then applies compression before generating the resized versions.
So if your original image is heavily compressed, those quality issues will carry over to the thumbnails as well. To fix this, try regenerating thumbnails from a higher quality original image.
It‘s also possible that the resized images are too small for their intended use. For example, trying to stretch a 150x150px thumbnail to fill a 500x500px space will look pixelated and blurry. Make sure you‘re using an appropriately sized thumbnail for each use case.
Can I delete unused thumbnail sizes?
Yes! If you have thumbnail sizes that are no longer needed (maybe from an old theme or plugin), you can absolutely remove them.
First, make sure the relevant add_image_size call is removed from your theme‘s functions.php file or the plugin that defined it. Then, you can safely delete the resized image files from your server.
Unused image sizes will be located in your /uploads directory, usually in a subfolder named after the size. For example, a thumbnail size named portfolio-large would be in /uploads/portfolio-large.
Just be very careful when deleting files from your server! Double-check that the thumbnails aren‘t still in use somewhere. When in doubt, make a backup first.
Wrapping Up
Phew, that was a lot to cover! I hope this deep dive into WordPress thumbnail sizes has given you a better understanding of how they work and how to optimize them for your site.
The main takeaways:
- WordPress generates resized versions of uploaded images called "thumbnail sizes"
- Themes and plugins can define their own custom thumbnail sizes
- Too many thumbnail sizes can bloat your media library storage
- Regenerate thumbnail plugins let you recreate sizes for existing images
- Lazy loading, WebP format, and compression can further optimize thumbnails
- Be careful when changing or deleting thumbnail sizes
Mastering thumbnails is a key part of keeping your WordPress site lean, fast, and visually awesome. With the techniques and best practices covered in this guide, you‘ll be a thumbnail wizard in no time.
Now go forth and optimize those thumbnails! Your visitors (and your web host) will thank you.