If you ever need to remove a photograph that has been glued to paper or cardboard, you can try using the same trick that stamp collectors use: soaking in water. Amateur photographer Michael T. Lauer writes on Quora,
Photos are processed in water so they can stay in water for a fairly long time. A lot of glue is not waterproof so it will lose strength in water. So, I’d approach this by soaking a print with paper backing in a tray with water (at room temperature) for 20-30 minutes. Take the print out of the water and lay it on a piece of rigid glass or plastic face down. Try to work the paper off the print by lifting at the edges. This part is trial and error.
After completing the work on the back, clean (squeegee) the glass/plastic and dip the print briefly in the water bath. Place the print on the glass face-up and squeegee the surface so that it’s free of water drops (this will prevent spotting). Place the print on a drying screen (a screen like what is used in a window but not metal) face down and leave it where air can circulate around it to dry over night.
Lauer warns against using heat or physical removal of the glue and paper, as both techniques could cause damage to the print.