Gform compatibility

add_action( 'gform_pre_submission', '__your_prefix__pre_submission_handler' );

function __your_prefix__pre_submission_handler( $form ) {

	$media_key = 'media_key'; // Need change on your media key

	$_POST[ $media_key ] = __your_prefix__get_image_id_by_url( $_POST[ $media_key ] );
}


function __your_prefix__get_image_id_by_url( $image_src ) {
	global $wpdb;

	$query = "SELECT ID FROM {$wpdb->posts} WHERE guid = %s";
	$id    = $wpdb->get_var( $wpdb->prepare( $query, esc_url( $image_src ) ) );

	return $id;
}