It is possible the server is running into a memory limit issue or the images being uploaded are exceeding the max file size limitation.
try adding the following at line 1 of the connections/powergallery.php file to increase the server memory limit, post max size and upload file max file size settings:
<?php
ini_set('memory_limit', '256M');
ini_set('upload_max_filesize', '10M');
ini_set('post_max_size', '10M');
?>