1/4" wood. I used a wood panel I found at Home Depot 2'x4' for about $7
Table saw/ Jig saw/ Circular saw - any kind of saw to cut your wood panel into 2 pieces
Make your photo you want to use and print 2 copies (I did one black&white, why waste colored ink). I used Paint.net, it's a free program you can download. Or you can use Photoshop, or if you don't want to mess with editing, you could just use a family photo instead of making a new scene. (For me this was the longest step, finding the pictures and putting it all together)
Spray the spray adhesive onto the back of your picture, and stick it in the middle of one of your panels.
Once everything is all Mod Podged together, get your second piece of paneling. Using wood glue, glue the 2 pieces together and set something heavy on top of the puzzle. Make sure your heavy thing covers the whole puzzle or else you might have a gap inbetween the 2 pieces. Let the glue dry.