How to change post thumbnail crop position in WordPress WITHOUT HACKING CORE

DISCLAIMER: This requires WordPress 3.4 to work correctly. The filter was added based on Trac Ticket #15989

Okay, to start, here is the function that we’re going to be working with, from ~/wp-includes/media.php :

/**
 * Retrieve calculated resized dimensions for use in imagecopyresampled().
 *
 * Calculate dimensions and coordinates for a resized image that fits within a
 * specified width and height. If $crop is true, the largest matching central
 * portion of the image will be cropped out and resized to the required size.
 *
 * @since 2.5.0
 * @uses apply_filters() Calls 'image_resize_dimensions' on $orig_w, $orig_h, $dest_w, $dest_h and
 *		$crop to provide custom resize dimensions.
 *
 * @param int $orig_w Original width.
 * @param int $orig_h Original height.
 * @param int $dest_w New width.
 * @param int $dest_h New height.
 * @param bool $crop Optional, default is false. Whether to crop image or resize.
 * @return bool|array False on failure. Returned array matches parameters for imagecopyresampled() PHP function.
 */
function image_resize_dimensions($orig_w, $orig_h, $dest_w, $dest_h, $crop = false) {

	if ($orig_w <= 0 || $orig_h <= 0)
		return false;
	// at least one of dest_w or dest_h must be specific
	if ($dest_w <= 0 && $dest_h = $orig_w && $new_h >= $orig_h )
		return false;

	// the return array matches the parameters to imagecopyresampled()
	// int dst_x, int dst_y, int src_x, int src_y, int dst_w, int dst_h, int src_w, int src_h
	return array( 0, 0, (int) $s_x, (int) $s_y, (int) $new_w, (int) $new_h, (int) $crop_w, (int) $crop_h );

}

Some people have looked at this and felt that there was no way to override the $s_y = floor( ($orig_h - $crop_h) / 2 ); line without hacking core. Well, that’s iffy, but if you scroll up above, you’ll see something that lets you basically ‘short-circuit’ the function in question:

// plugins can use this to provide custom resize dimensions
$output = apply_filters( 'image_resize_dimensions', null, $orig_w, $orig_h, $dest_w, $dest_h, $crop );
if ( null !== $output )
	return $output;

So here’s our function to make all crops start at (0,0) — customize it as you like:

function my_awesome_image_resize_dimensions( $payload, $orig_w, $orig_h, $dest_w, $dest_h, $crop ){

	// Change this to a conditional that decides whether you 
	// want to override the defaults for this image or not.
	if( false )
		return $payload;

	if ( $crop ) {
		// crop the largest possible portion of the original image that we can size to $dest_w x $dest_h
		$aspect_ratio = $orig_w / $orig_h;
		$new_w = min($dest_w, $orig_w);
		$new_h = min($dest_h, $orig_h);

		if ( !$new_w ) {
			$new_w = intval($new_h * $aspect_ratio);
		}

		if ( !$new_h ) {
			$new_h = intval($new_w / $aspect_ratio);
		}

		$size_ratio = max($new_w / $orig_w, $new_h / $orig_h);

		$crop_w = round($new_w / $size_ratio);
		$crop_h = round($new_h / $size_ratio);

		$s_x = 0; // [[ formerly ]] ==> floor( ($orig_w - $crop_w) / 2 );
		$s_y = 0; // [[ formerly ]] ==> floor( ($orig_h - $crop_h) / 2 );
	} else {
		// don't crop, just resize using $dest_w x $dest_h as a maximum bounding box
		$crop_w = $orig_w;
		$crop_h = $orig_h;

		$s_x = 0;
		$s_y = 0;

		list( $new_w, $new_h ) = wp_constrain_dimensions( $orig_w, $orig_h, $dest_w, $dest_h );
	}

	// if the resulting image would be the same size or larger we don't want to resize it
	if ( $new_w >= $orig_w && $new_h >= $orig_h )
		return false;

	// the return array matches the parameters to imagecopyresampled()
	// int dst_x, int dst_y, int src_x, int src_y, int dst_w, int dst_h, int src_w, int src_h
	return array( 0, 0, (int) $s_x, (int) $s_y, (int) $new_w, (int) $new_h, (int) $crop_w, (int) $crop_h );

}
add_filter( 'image_resize_dimensions', 'my_awesome_image_resize_dimensions', 10, 6 );