They were so easy and so popular with the friends who came to the house and saw them that I am now selling them through one of the local comic book stores.
They're really easy too, can be done with any images and use supplies that can be purchased at any craft store.
I start with clear flat marbles like these (lots of people put them in the bottom of fish tanks)
I then use the marbles to scan for the right image and trace them.
It is important to individually trace each image because the marbles vary in shape and size. After tracing I cut them out on my cutting matte with my exacto-knife. Because the marble curves in from where I trace I usually make my cut about 1/8" in from the line I drew.
I laminate the image onto the marble with elmers glue.
Attach the magnet to the back with a heavy duty adhesive.
I'm pretty happy with how they turned out.
You can make them with any images, but I found that the comic books were working by far the best. Clean lines, bright colors and interesting shapes. If I were to do anything else I would probably start looking at brochures, magazines or other media with high quality images.