As with sidebars and navigation menus, we can’t just output featured images in our theme and expect them to work, we must first tell WordPress that we support the feature. In functions.php add:
add_theme_support( ‘post-thumbnails’ );
Now we can add_post_thumbnail(); in our loop and the thumbnails will work. The problem is that they will be output in WordPress’ large size of 1920px x 2560px, which is too large for most purposes. Luckily WordPress has another helper function: add_image_size();
When a user uploads an image, if an image size is defined, WordPress will generate a version of the uploaded image at that size (while keeping the original image). If the user’s image is smaller than the size you set, WordPress will not do anything because it can’t make the image larger than the original.
To use the optimized feature image instead of the original image, place the following code in functions.php:
add_image_size( ‘my-custom-image-size’, 640, 999 );
The first and second parameters are the image width and the third is the height. Both height and width are optional if you only want to restrict one dimension.
Call the tag in index.php:
the_post_thumbnail( ‘my-custom-image-size’ );
