So, let’s imagine that your website has 10 registered custom post types, and each of them use 2-3 image sizes on the website pages. It is easy to understand that when we upload any picture on the website WordPress creates 20-30 copies of it!

There is no way to use add_image_size() for a custom post type but here is a small piece of code that solves this problem. This code tells WordPress when it have to create a certain resolution copy of the picture and when it doesn’t.

Both intermediate_image_sizes and intermediate_image_sizes_advanced hooks are OK for this task. A super simple example below, after you insert the code to your functions.php file thumbnails my-image-size-name won’t be created for images, uploaded from custom post type page.

add_filter( 'intermediate_image_sizes_advanced', function( $sizes ){ 
	if( isset( $_REQUEST['post_id'] ) && 'page' == get_post_type($_REQUEST['post_id'] ) ) {
		unset( $sizes['my-image-size-name'] );
	} 
	return $sizes; 
} );

A little bit more complicated example, but it is also correct:

/*
 * this hook will be fired while you uploading a picture
 */
add_filter( 'intermediate_image_sizes', 'misha_reduce_image_sizes' );
 
function misha_reduce_image_sizes( $sizes ){
	/*
	 * $sizes - all image sizes array Array ( [0] => thumbnail [1] => medium [2] => large [3] => post-thumbnail )
	 * get_post_type() to get post type
	 */
	$type = get_post_type($_REQUEST['post_id']); // $_REQUEST['post_id'] post id the image uploads to
 
	foreach( $sizes as $key => $value ){
 
		/*
		 * use switch if there are a lot of post types
		 */
		if( $type == 'page' ) {
			if( $value == 'regionfeatured' ){ // turn off 'regionfeatured' for pages
    				unset( $sizes[$key] );
    			}
		} else if ( $type == 'region' ) {
			if( !in_array( $value, array('regionfeatured','misha335') ) ){ // for regions turn off everyting except 'regionfeatured' and 'misha335'
    				unset( $sizes[$key] );
    			}
		} else {
			if( $value != 'thumbnail' ){ // turn off everything except thumbnail
    				unset( $sizes[$key] );
    			}
		}
	}
	return $sizes;
}
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