I just got off a support call WA.. the results of which was a new helperphp.php file. I am not sure if WA has updated the 'fixed' file that is in the transparent PNG background sticky in this forum or not, so I have attached the new one.
It will need go into the same place as the one listed in the sticky about PNGs and transparent backgrounds in this forum. It will also need to go into your "SITE_FOLDER/webassist/file_manipulation/" folder and be uploaded to your webserver.
Hope this helps.