RB External

gistfile1.txt

add_filter( 'get_post_metadata', function ( $value, $post_id, $meta_key, $single ) {
    if ( $meta_key === '_thumbnail_id' ) {
        $url_thumbnail_external = get_post_meta( $post_id ,'thumbnail_external', true );
        if( !empty($url_thumbnail_external) ){
            return "ext-" . $post_id;
        }
    }
    return $value;
}, 10, 4);

add_filter( 'wp_get_attachment_image_src', 'rb_external_attachment_image_src', 10, 4 );
function rb_external_attachment_image_src( $image, $attachment_id, $size, $icon ){
    if(isset($attachment_id) && !empty($attachment_id) && substr($attachment_id, 0, 3) === "ext"){
        $id = substr($attachment_id, 4);
        $thumbnail = get_post_meta( $id ,'thumbnail_external', true );
        return Array ( $thumbnail, 0, 0, false );
    }
    return $image;
}