I guess this happens when people frame prints with no mat so the print presses against the glass. I might try a little heat. Take a hair dryer and try directing the heat on the glass side. Not too much heat at first and try carefully peeling the print off starting at a corner.
You know when you buy products with those sticky price and product labels that leave a sticky,messy residue when you take them off? I use the heat from the hair dryer to carefully peel them off without tearing the label or leaving a residue. Maybe that will work.