I made several glass marble magnet sets this year as gifts and I can't believe I didn't try doing this sooner. Finding images in magazines and placing them under the flat glass marble was pretty addicting. I seriously couldn't stop. In fact, I cut out way too many circles and ran out of magnets. I highly recommend getting a 1/2" circle puncher so you get perfect circles and don't have to cut them by hand.
Since everyone with an internet connection has seen these marble magnet crafts before (they were the hot crafty project several years ago) I'll just include basic instructions. You need magnets, 1/2" flat glass marbles (I got mine at Michaels in the plant department), 1/2" circle punch, magazines and glue. Cut out circles in magazines and glue onto the flat back of the glass marble. Smooth out the glue. I used Tacky Glue and while it looks white initially it dries completely clear overnight. Then glue on the magnet. Let dry overnight.