Those purposes are system-defined. You can't add different ones. However, you could consider using the Image Description field (System > Master Information > Images) to get what you're looking for.
Please visit: http://www.xtuple.org/phpBB3/viewtopic.php?f=&t=3495&p=0 for the changed document
Yes, of course. There are many opportunities for you to change or add image item purposes. Good luck!