Hook documentation for the image_downsize filter

media.php

<?php

/**
 * Override default image downsize rules with a custom resizing service
 * 
 * Defaults to false (no third-party downsizing), but can pass an indexed
 * array of image details to use in place of WP's default downsizing rules
 * and short-circuit the remainder of the function.
 *
 * @since 2.5.0
 * 
 @param false|array    $image_downsize {
 *     Indexed array of details on success.
 * 
 *     @type string  $img_url The URL of the downsized image
 *     @type integer $width The width of the downsized image
 *     @type integer $height The height of the downsized image
 *     @type bool    $is_intermediate True if $img_url is a downsized image, false if it is the original
 * }
 * @param integer      $id   Attachment ID for image
 * @param array|string $size Size of image, either array (e.g. array( 120, 120 )) or string (e.g. 'medium')
 */
if ( $out = apply_filters( 'image_downsize', false, $id, $size ) )
	return $out;