I’d find someone with a good scanner, and get a good scan of it before trying anything. This way you’ll have a good photoshoppable image in case its gets screwed up trying to remove it from the glass.
There is a good chance that some of the color will stick to the glass when you pull it off, hopefully not that much though.
Anyway, here is a thread on the topic:
http://www.retouchpro.com/forums/history-conservation-repair/1664-removing-photos-stuck-glass.html