If the picture is of value to you and damage is an issue, contact a certified picture framer for help. If not, soak the glass picture side down in a tray of distilled water. The time will vary depending on how "stuck" the picture is and the finish on the picture. Photographs are the safest and easiest generally taking about 10 minutes to a half hour. Lithographs (Posters, reprints, etc..) are more problematic. To remove them you will also need a piece of cheese cloth under the print while it soaks to support the paper when you remove everything from the bath.
In either case, gently remove everything from your bath and with the glass facing up and your print resting on a clean towel, use a very thin wide blunt instrument to ease the picture slowly away from the glass. I've found that a burnishing bone, plastic or wood cuticle depressor or a wooden tongue depressor lightly sanded around the edge works best. Take your time. Once removed, let your picture rest face up on the towel until lightly damp, from there you will need to apply weight evenly over the entire picture to prevent ruckling. Generally, a piece of matboard slightly larger works.
Hope this helps.