What type of glue or adhesive is best to stick photos to a painted wood surface and what is the best thing to use to put a clear varnish over top of the photos?

I've recently built a bookcase for my child and we covered the bare plywood with various photos. We used a water based sealer/glue called "Mod Podge" manufactured by Plaid Enterprises Inc.

We found it very versatile in the sense that it can be used to glue the pix to the wood and then used on top to seal the pictures as well. Easy to apply, cleans up with water, non toxic. The label says that a clear acrylic sealer can also be used on top for added protection.