Whatever shape you decide on, cut two of them from your cardboard.
Cut a 1/4" thick piece of styrofoam to go between the two pieces - make it approximately the same shape as your cardboard, but a little bit smaller. Glue the pieces together, centering the styrofoam between the cardboard.
Paint the entire thing white or whatever color you want. Allow the paint to dry before you start decoupaging.
If you want, cut wrapping paper to cover the back and front of your ornament. This ornament is covered with a shiny white paper saved from last year. Glue the paper onto the cardboard, attach your other pictures and cover both with a coat of decoupage medium. Allow to dry completely.
Tie a pretty ribbon around the rim of the ornament. You can tie a loop in the ribbon and use it to hang the ornament, or just tie a bow and insert a wire hook under it to hang.