After fighting like crazy I finally managed to hack something together... I couldn't touch the plugins files or the function file (and I´m not really good at PHP). BUt I managed to strip everything from the code that get_the_image gives you and leave the actual image URL.
What I wanted to do here is to have a direct link to the image (instead of a link to the page where the image is). I'm sure there are better methods but this is mine.
$text = get_the_image( array( 'custom_key' => array( 'thumbnail' ), 'echo' => false, 'link_to_post' => false,'size' => 'full') ); ;
$matches = $matches;
$list = array();
foreach($matches as $var)
print("<a href='".$var."' rel='lightbox'>");
<li class="post-<?php the_ID(); ?> thumb-big"><?php get_the_image( array( 'custom_key' => array( 'thumbnail' ), 'link_to_post' => false,'default_size' => '320x320', 'width' => '320', 'height' => '320' ) ); ?></li></a>