get_imag_id

<?php

/**
* Get the attachment ID of an image from it's URL.
*
* @param string $image_url The URL for the image you need an id for.
*
* @return mixed
*/
function theme_get_image_id($image_url) {

    global $wpdb;

    $attachment = $wpdb->get_col($wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE guid='%s';", $image_url ));
    return $attachment[0];

}