Not that I would ever recommend that you even try this, but - I have removed stuck photos from glass (these are the older snapshot kinds - haven't tried paper backed or digital papers) by submersing the photo and glass (photo side up, glass side down)into a large pan of water. The photo will often float off the glass on its own. After the photo is off the glass, remove it from water and let it dry. Color photos will be really bizare colors, but dry back to normal. Check first to be sure that the photo does not have any tears in the emulsion in it, and don't do this if it does.
I have also done this with "antique" prints bought at the second hand store or garage sale (which my customers love to shop at). After the print comes up, I gently wipe off as much moisture as I can. I put it between two sheets of rag board and stick it in the vacuum press and let it smash out the moisture. Change the boards a few times and keep doing this until the paper is almost dry. Then let it air dry on a dry rag board. I have also done this with one of those tourist papyrus that a customer drove through a car wash with - windows of the car down during the wash.
A customer brought in a print on a hot day that was the low quality "canvas" stuff that has a sprayed on coating instead of a lamination film. It had stuck to her purse (which she had plopped on top of it) in the heat of the car while she was driving to the store. I stuck the picture and the purse in the refrigerator. When it cooled off, the purse could be removed. As soon as the picture warmed up, the coating became sticky again.
Not that I would EVER suggest that you do these things, and if you EVER do, be sure that the art is not valuable in any way, and that the customer understands that it might not work, and that the safest, bestest thing to do would be to contact someone such as a conservator.
As Phill pointed out, there is a big risk in trying these things. Sometimes it doesn't work.
