I have a series of posts, all with featured images, but I need to be able to customise the crop top right corner. In this instance, I need them to be cropped from top right, but it would be useful to also know how to position that point myself.
At present, the add_image_size() function is taking its crop from the centre of the image. Not always pretty!!
Intermediate image generation is extremely rigid. image_resize() keeps it close to code and completely lacks hooks.
Pretty much only option for this is to hook into wp_generate_attachment_metadata and overwrite WP-generated image with your own (which will need bit of a image_resize() fork).
I need this for work so I might be able to share some code later.
Ok, here is rough, but working example. Note that setting up crop in this way requires understanding of imagecopyresampled().
add_filter('wp_generate_attachment_metadata', 'custom_crop'); function custom_crop($metadata) { $uploads = wp_upload_dir(); $file = path_join( $uploads['basedir'], $metadata['file'] ); // original image file list( $year, $month ) = explode( '/', $metadata['file'] ); $target = path_join( $uploads['basedir'], "{$year}/{$month}/".$metadata['sizes']['medium']['file'] ); // intermediate size file $image = imagecreatefromjpeg($file); // original image resource $image_target = wp_imagecreatetruecolor( 44, 44 ); // blank image to fill imagecopyresampled($image_target, $image, 0, 0, 25, 15, 44, 44, 170, 170); // crop original imagejpeg($image_target, $target, apply_filters( 'jpeg_quality', 90, 'image_resize' )); // write cropped to file return $metadata; } 
image_resize function. Rarst was making the point that you'd have to hook into the resizing process, but create the image sizes yourself manually. Wordpress codex has the answer, its below.
Set the image size by cropping the image and defining a crop position:
add_image_size( 'custom-size', 220, 220, array( 'left', 'top' ) ); // Hard crop left topWhen setting a crop position, the first value in the array is the x axis crop position, the second is the y axis crop position.
x_crop_position accepts 'left' 'center', or 'right'. y_crop_position accepts 'top', 'center', or 'bottom'. By default, these values default to 'center' when using hard crop mode.
And also codex references a page which shows how crop positions acts.
http://havecamerawilltravel.com/photographer/wordpress-thumbnail-crop
I have developed a solution to this problem that does not require hacking the core: http://bradt.ca/archives/image-crop-position-in-wordpress/
I have also submitted a patch to core: http://core.trac.wordpress.org/ticket/19393
Add yourself as a Cc on the ticket to show your support for it to be added to core.
You can use the plugin Thumbnail Crop Position to select the crop position of your thumbnails.
Alternative solution here: http://pixert.com/blog/cropping-post-featured-thumbnails-from-top-instead-of-center-in-wordpress-with-native-cropping-tool/
Just add this code to functions.php, then use "Regenerate Thumbnails" plugin (https://wordpress.org/plugins/regenerate-thumbnails/):
function px_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', 'px_image_resize_dimensions', 10, 6 ); 