You could always have a copy of the photo laminated by an office supply store. They should have a few samples of different thicknesses so that you can find one that works for you. Once you have your photo protected by the laminate, round off the corners and you're good to go!
Quick note about cutting anything laminated, generally there's a little cushion of air around the original item. You find this a lot with thicker papers. So when you're trimming, make sure you don't cut into this cushion. Your soap could leak in and destroy the photo.