So here's how I made them:
First, I went to Goodwill and bought a few cheap, framed pictures. (The ones I bought were each less than $5.) I tried to choose ones that had "real wood" frames so they would last longer.
Next, I removed all of the artwork, glass, staples, and paper, so I was left with empty frames.
Then it was time to paint the frames. I used one color of semi-gloss spray paint to help unify the variety of the frames' shapes and sizes. I propped up the frames on some wooden blocks and sprayed two coats onto the fronts and edges. For a more "blingy" look, try using a high-gloss paint--I think a glossy black on one of those fancy guilded frames would look totally cool.
After the frames were dry, I hot-glued magnets around the backside of the frames. You need some strong magnets for this. I used thick, disc-shaped magnets that have a strength rating of "6."
You can buy packs of 50 of these magnets at craft stores like Hobby Lobby. The number of magnets you'll need for one frame depends on its weight, but I ended up using at least a dozen magnets per frame.
Don't forget to stop by my blog, The Thinker Builder. Have fun building!