At one time or another, many of us will run across a photo that's been in a too-humid environment and is stuck to the glass in a frame.
Whether it can be removed safely depends on the emulsion and the paper, and what works for one person might not work for you.
Here's what to try.
First, and critically important before attempting any removal, either scan or photograph the image through the glass. Taking a photo of it with a quality digital camera may be best, so you can focus through the glass' thickness. Be careful to notice and adjust the lighting on the image, so you don't create shadows or glare. It can be difficult to photograph it at a good angle, though, because of the reflection in the glass.
If you scan the original, do so at 600 dpi (dots per inch).
The goal is to have a quality backup that you can use to edit and reprint, in case you cannot save the original. Don't just depend on the thumbnail view of the scan or photo you take; download it and review the digital file carefully to make sure it's the best quality, since trying to remove the original print from the glass risks that it will be ruined. Once you've downloaded the image to your Digital Photo Hub, back it up in multiple locations.
After you're sure you have a quality digital backup, you can attempt to remove the image.
In the future, be sure to use an archival quality, photo safe mat between any photo and the frame to act as a buffer for humidity. And store photos the same places you like to live - a bedroom, office or living room - not an attic or basement unless it's temperature and humidity controlled.
Some people may choose to stop here with their digital copy and just turn that into a new print. But others, depending on the value of the photo, may want to find a photographic conservator to determine the type of print and degree of damage to the original, or try to make repairs themselves.
If the photo is extremely valuable, a conservator would be the best way to go. Find one through the American Institute for Conservation of Historic and Artistic Works or the Regional Alliance for Preservation.
For you Do-It-Yourself types, you may have success with heat, cold, or water.
Heat: Take a hair dryer on the lowest setting and hold about five inches away from the back of the image, blowing the warm air on the print to see if it releases from the glass.
Cold: Carefully wrap the glass and photo in paper and use clean gloves in case the glass breaks. Freeze for an hour. See if the image will peel up. You may need to use some dental floss like we do when removing images from "magnetic" i.e. sticky albums .
Water: For newer photos that have not been stuck long, soak in plain water. For ones that have been stuck longer, use a quarter-teaspoon of Kodak PhotoFlo solution per cup of water, and let the image soak. PhotoFlo is used in the final stage of processing photos, and it may help release the image. If the photo does release using this process, do NOT use heat to dry it. Lay it image-side-up on a clean, flat surface so it can dry on its own.
There are no guarantees with any of these methods, but they're ways to approach the problem. Just make sure you've got that quality digital backup - photo or scan - first.