Function to get custom post image URL

I have to retrieve the URL from a custom post image, just the URL, like: http://ift.tt/1MWIQMj.

The code which I have to filter is this:

 <?php cp_get_image_url(); ?>

which returns this:

<a href="http://ift.tt/183xSE2" class="img-main" data-rel="colorbox" title="Chevrolet A10"><img src="http://ift.tt/1MWITrg" title="6" alt="6" /></a>

I tried some functions using wp_get_attachment_image_src with no success.

Example:

<?php $image = wp_get_attachment_image_src(  cp_get_image_url() ); echo $image[0]; ?>