[Plugin: NextGEN Gallery] Editing Thumbnails

HI

I am unable to adjust the view shown by thumbnails in my gallery. If I try to edi the thumbnail, I am able to add the extending square and to reposition it but when the UPDATE button is pressed I get a message that an error has occurred, Error Updating Thumbnail. So should I change something by way of ftp/CHMOD? At present I have 755 CHMOD for my folders, but which ones should I alter to allow the edit to take effect?

Andy