These are the two products I use to secure and seal my photographs when creating my photo jewelry.
They can each be found at most national craft retailers or online.
The Glossy Accents is thick and great for filling in a "well" or covering small beads, findings or paper layers. It has it's own applicator and can fit in small areas. A very tiny amount can be applied to the back of your photograph to secure it to the piece of jewelry before covering the top.
The Triple Thick is thinner and easier to spread over larger surfaces. I use a clean, soft, synthetic craft brush to apply it. It gives a uniform look to a wider surface. You must take care not to over-brush the top or your photo might smear.
I also sometimes use a spray fixative (found at any art store) to "fix" or seal my photographs or paper art before finishing with one of the above products.
Be sure to experiment with any product you use and get comfortable with it. There can be a slight learning curve and it will make for happier creating if you feel secure with your supplies.