getIDfromGUID.php

<?php

//Locate the attachment post's ID in the database based on the GUID.
function getIDfromGUID( $existing_image ){
    global $wpdb;
    return $wpdb->get_var( $wpdb->prepare( "SELECT ID FROM $wpdb->posts WHERE post_type=attachment AND  guid=%s", $existing_image ) );
}


$existing_image = "http://guidevoyage.org.dev/wp-content/uploads/2012/02/Fontaine-de-Trevi.jpg";
var_dump( getIDfromGUID( $existing_image ) );