WordPress Image Size – How to Choose The Right Image Size

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Here at WPrBlogger, I’m obsessed with website performance. My routine includes optimizing for speed, Core Web Vitals, and overall performance. 

Among those website chores is WordPress image size optimization.

In WordPress, optimizing images isn’t just about aesthetics—it impacts site speed, SEO, user experience, and overall site functionality. 

Choosing the right image sizes and ensuring they are optimized for your WordPress website can make a significant difference in performance. 

This guide covers everything you need to know about WordPress image sizes and how to optimize them effectively.

Default WordPress Image Sizes

WordPress automatically generates four image sizes when you upload an image file, such as JPEG or PNG, to your site. 

Understanding these default sizes will help you choose the right ones for your content:

• Thumbnail (150×150 pixels) – Used in widgets or galleries.
• Medium (300×300 pixels) – Often used for in-content images.
• Large (1024×1024 pixels) – Best for blog post headers or larger images.
• Full size – The original dimensions of the image as uploaded.

Each of these serves different purposes, and selecting the appropriate size is essential for maintaining a balance between quality and performance.

For example, WordPress uses different image sizes for post-featured images, related post widgets, and other widgets that display published posts. The blog post featured or header image is mostly large (1024 x 1024 pixels), while related posts use a thumbnail size (150×150).

However, you can customize these settings if your theme allows it. We will get to that later. 

Custom WordPress Image Sizes

While WordPress provides default sizes, you can create custom image sizes tailored to your needs. This is particularly useful for themes or specific content requirements.

There are a few ways you can customize the image size in WordPress.

First, go to WordPress admin and hover over Settings from the menu list. Click “Media” from the options. 

On this screen, you can change the default WordPress Image size to suit your specific needs. 

Remember, the option you set here is universal, meaning it affects sitewide image size. Unless you change an image size setting at the page level, the option you input here will take effect. 

Another way to add custom image size in WordPress is to modify your theme’s functions.php file.

But this option is not advisable for beginners. However, for learning’s sake, here’s a PHP code to customize the size of images in WordPress. 

add_image_size('custom-size', 600, 400, true);

This code creates a custom size of 600×400 pixels with cropping enabled. It’s important to use the right dimensions based on where the images will appear (e.g., blog thumbnails or featured images) to ensure consistency across your site.

Once more, using the Media setting is more straightforward and beginner-friendly. I advise you not to try the theme modifications option if you’re not a skilled programmer. 

Optimizing Image Sizes for Performance

Apart from resizing images to fit their container and make them visually more appealing, optimizing WordPress image size for performance is also important. 

Large images can slow down your website, negatively affecting user experience and SEO. 

You must ensure images are properly sized to fit the container and reduce file sizes without compromising quality. The size of your images must be within the expected dimension for the browser to load it quickly.  

Here are some ways to optimize WordPress images for performance. 

• Compressing Images – Compressing image size to make it smaller is a good technique to improve loading time, especially for mobile users. Use tools like TinyPNG or Imagify to reduce file sizes.
• Plugins for Optimization – WordPress plugins like Smush, Imagify, and ShortPixel automatically compress images as you upload them, ensuring optimal performance.
• JetPack Accelerator – Use JetPack Site Accelerator to automatically resize and server images from the JetPack global CDN and convert them to WebP format for supported browsers. 

By keeping image sizes small, you’ll improve your site’s speed while maintaining visual appeal. The JetPack image CDN is a good option to consider if you’re not using a CDN like Cloudflare.

By serving images from a global content delivery network, JetPack Site Accelerator ensures images are served from the closest server to the user. This will significantly improve page load time, website performance, and user experience. 

How to Discover The Right Image Size in WordPress

Sometimes, you take a screenshot of web pages or photos and want to use them in your WordPress sites. These images or photos are normally of larger size. 

How do you know if these images or photos are of the expected dimension and help improve page speed? 

This is where the JetPack Image Guide comes in. 

JetPack Image Guide is a simple, yet powerful tool that let you see images that are too large or snall on your site when you’re loggedin as admin. This feature is helpful for ensuring you’re adding the correct image size to your WordPress site. 

JetPack Image Guide is part of the JetPack Boost suite of tools; you can enable or disable it at will. To use the feature, install and activate the JetPack Boost plugin and enable the Image Guide option. 

Once active, visit your site as admin and navigate to the page on which you want to investigate its images. As you browse through the page/post, watch out for the Image Guide bubble on every image on that particular page/post. 

Next, move your mouse cursory on the bubble to reveal the Jetpack Image Guide card with details about the image size and optimization recommendations. 

This card has some nice details worth attention:

• Image File Dimension
• Expected Dimension
• Sie on screen
• Image Size
• Potential Saving
• And links to learn more about image optimization. 

Following the information in the Image Guide, you can discover the right image size to upload and how best to optimize it for faster loading and optimal user experience. 

Note, consider viewing the site on different devices because the JetPack Image Guide shows the details based on the current viewing devices. Each device might have different results and details. 

For example, viewing the above post on a mobile device shows different performance results. 

This is one of the reasons it is best to use an automatic image resize tool like JetPack Site Accelerator or Imagify. So you don’t have to worry about resizing images for different screen sizes. 
Check this article to learn more about how the JetPack Image Performance Guide works.

Serving Responsive Images in WordPress

In WordPress, responsive images are a built-in feature. They allow your site to serve different image sizes depending on the user’s device.

Whether it’s a mobile device or a desktop computer, WordPress automatically generates responsive image sizes on the fly without affecting image quality. Also, it doesn’t matter what browser type, Chrome, Microsoft Edge, Safari, or Firefox; the images are automatically downloaded and rendered accordingly. 

If you review the above images (mobile and desktop) in the how-to discover the right image section, you will notice the difference in the image details. This is because WordPress serves responsive images automatically based on the user’s device screen size. 

Anyway, here are some ways WordPress displays responsive images for your users. 

• Srcset – WordPress automatically generates multiple sizes of each image and uses the srcset attribute to display the appropriate version based on screen size.
• Sizes Attribute – This tells the browser what size the image should be displayed at, improving load times on smaller screens.

With this functionality, your images will load faster and look great on any device.

Best Practices for Choosing the Right Image Size

Selecting the right image size for each type of content is key. You can save significant kilobytes (KB) just by serving the right image size in WordPress. 

In addition, website performance is key; serving the right image size will reduce Cumulative Layout Shift errors in Page Speed Insights and other Web Page testing tools. Also, it will improve the loading of critical image assets on the page because web browsers will have no difficulty in determining and downloading the appropriate image size quickly. 

Here are some recommendations for best image size practice:

• For Blog Posts – Use medium-sized images (e.g., 300×300 or 600×400) to balance quality and load time. This is for images within the blog post body. 
• For Featured Images – Large images (1024×1024 or higher) work best for full-width headers or sliders. Check your theme for the best-recommended size. Many WordPress themes work best with 1200×657 pixels. 
• Thumbnails – Don’t reinvent the wheels; stick to the default 150×150 size for gallery views or preview images.
• Avoid Oversizing – Always resize images before uploading, rather than relying on WordPress to handle resizing, to avoid unnecessary large file uploads. As said earlier, use Imagify online tools to resize images before uploading them to WordPress.

Conclusion

Optimizing image size and performance in WordPress isn’t just a technical task—it’s essential for improving your site’s performance, user experience, and SEO. 

By understanding the default image sizes, adding custom sizes when needed, and utilizing optimization tools, you’ll ensure that your WordPress site looks great without sacrificing speed.

I have shared sufficient details and helpful tips with you so that when you implement the tips, you can discover, resize, and set the best WordPress image size for your website.