Before doing anything else, I would suggest making as high a quality and high resolution scan (2400dpi not interpolated) as you can using a flatbed scanner and save the image in an uncompressed file format like TIF, not JPG. Do that through the existing glass before doing anything to recover the photo. The file will be huge, but not an issue for current storage technology.

The image is the most important thing. Being able to make a reproduction, even imperfect, is going to be better than loosing the whole thing in a recovery attempt.

If the upper portion is not adhered, then I would just cut it free and resize as a 2"x3" or wallet. Once you soak the paper, it wont lay flat.

If the upper portion of the image is adhered as well as the lower, and the substance is water soluble, Then Alan's suggestion will work. If you want to test it, only soak the lower portion of the image and see how well it separates.

I think it is more likely to be the plastic matte coating over the dye layer that is sticking and maybe from heat and preasure. Before trying water I would carefully try a sharp straight razor starting from the bottom and see if you can cut them apart.

If what is sticking them together is water soluble then Alan's suggestion will work fine. If say it got really hot, or the substance is oil based then soaking will lift the paper off of the dye layer instead of the dye layer off of the glass. The dye layer and matte surface are not strong. the paper is the strength of the image.