Hi Catherine - You could try: Go to Manage Store, then under side bar on right, select global preferences. It doesn't show a link to edit existing card image by text but the link above for watermark preference has a contact GCU and perhaps if you type in your revised image by text and give your request in writing to  GCU tech support that you would like all your cards to be revised, they can do existing cards in bulk vs. a time consuming one at a time.
We should put bulk editing on our wish list AND while I'm at it, it would be nice if we could select multiple cards at a time for rearranging store card orders too.
Cheers, Lisa Charlton