I would think maybe with a light steam. I stress "light" because you do not want to ruin the picture.

I have also had this happen from humid conditions where I live. I do not know of a remedy. When I tried to remove the photo, it tore... I will be watching this question to see what others might have to offer.

The first answer is right about the scanning. It is the only way to go. Besides doing it yourself you can also take it to a good photo lab and let them work their photoshop magic. So you know, the photo gets stuck to the glass because moisture has gotten trapped between the glass and the photo. This often happens when someone sprays the framed picture with glass cleaner and it drips down and seeps und the glass. But it can also happen from just being in a damp or steamy environment. There are two things you can do to prevent this in the future. First, never spray the glass directly, spray the cleaner onto your rag or paper towels and then wipe the glass clean. Second, change the glass out for plexiglass, 1/8" framer's quality not the thin styrene stuff, as it is porous and allows air to pass through thus preventing moisture from getting trapped. Also, photo frames generally come with glass and cardboard. If you really care about your picture, swap out not only the glass for plexi, but the cardboard for an archival/acid free board. Cardboard is an acid factory and eventually destroys any paper that it touches. If you're wondering, I've been a picture framer for almost twenty years and I see these problems a lot.