This is super easy and uses all materials that you have lying around already- mostly stuff that would have been garbage anyways. All this takes is some plastic bottles, cardboard, paint, and glue. Super easy!
Step 1: Cut Some Plastic Shapes
Step 2: Make the Cardboard Frame.
Cut four strips of equal width cardboard- I measured at two points and cut with a straight edge. Then lay the first two edges against two edges of the picture and fold one end over the other one. Tape the edges together and move on to the next corner and repeat until all four corners are taped together. At this point check your fit and you should have kind of shadowbox cardboard frame that fits the picture tightly.
Glue the overlapping edges of the cardboard together and make sure the edges are still well taped together to keep the glue in place while it dries.
Step 3: Paint the Plastic Shapes
Paint all the shapes white first- I use acrylic paints but this project would be well suited to spray paint, although the downside is the drying time is longer with spray paint- acrylic dries fast unless you add retarder to it. House paint would work for this fine if you have that lying around- basically whatever you've got, use it up!
Then paint the shapes in colors that will look good with the painting- probably colors that already are in there- this is where real acrylic paints come in handy because you can mix them to make any color you want. I actually mix my acrylic with Elmer's glue as a cheap medium to help it apply more smoothly and make the paint spread farther, and it also makes blending colors easier to do, also gives a nice even gloss to the finish. My general mix is about 2/3 glue to 1/3 paint but you can use less or more depending how opaque you want it.