How to Add Custom Image Sizes to WordPress Uploader

WordPress 3.3 is finally out, and features a host of new improvements in the UI, along with other novelties.

A new feature I’m digging is the image_size_names_choose filter, which enables us to add new image sizes to the media upload/insert interface.

Check out the code below and the resulting screen:

if ( function_exists( 'add_image_size' ) ) {
    add_image_size( 'new-size', 300, 100, true ); //(cropped)
}

add_filter('image_size_names_choose', 'my_image_sizes');
function my_image_sizes($sizes) {
        $addsizes = array(
                "new-size" => __( "New Size")
                );
        $newsizes = array_merge($sizes, $addsizes);
        return $newsizes;
}