Responsive Cropped Images in WordPress

WordPress 4.4 added support for srcset and sizes, allowing the browser to pick the best version of an <img> from a list of different sizes, which we covered in depth earlier this year. WordPress generates all of these different sizes when an author uploads an image, in addition to any custom sizes added by plugins or the current theme.

While implementing a full width featured image in a recent project, I ran into a confusing bug where srcset wasn’t being generated for the custom image size added by our theme, resulting in a blurry image on high res devices and a much too large image on mobile. This size (named header-thumb) is 1170×400 pixels and is configured to crop the original image if it’s not the same aspect ratio.

After spending quite some time reviewing the documentation, I stumbled across a thread on the WordPress developer blog that explained everything. Unfortunately, this wasn’t documented anywhere I could find on WordPress.org.

Since there are no other thumbnails matching that aspect ratio, we need to add additional cropped sizes to compensate.

functions.php

Before

add_image_size('header-thumb', 1170, 400, true);

After

add_image_size('header-thumb-small', 585, 200, true); add_image_size('header-thumb-medium', 804, 275, true); add_image_size('header-thumb-large', 1170, 400, true); add_image_size('header-thumb', 2340, 800, true);

Now, wherever our theme calls the_post_thumbnail('header-thumb'), we get an <img> tag with the above sizes in its srcset.

Note that the header-thumb size was doubled to account for larger high DPI displays. Also, you’ll need to regenerate your media library’s thumbnails with the Regenerate Thumbnails plugin or wp-cli if you have existing images.