I don't know if this would be considered a "correct" way of doing it, but its what I did:
1. Find a small image (mine came from teeny magazines that I stole from my little sister), and place a marble over it, tracing around it with a pen.
2. Cut out the image.
3. Cover the under (flat) side of the marble with plain ol' Elmers glue...I just put some on my finger and rub it on.
4. Place the picture onto the back of the marble.
5. Allow to dry, and add magnet. (I used the adhesive magnetic sheets and just traced and cut like I did with the picture.)
6. Trim off any access picture or magnet that sticks out from around the edges of the marble.
Its really quite simple.
Just a comment about the Elmer's... if you want these to last a really long time, I would find some glass glue! I started out using "Tacky" craft glue, and a bunch of my marbles fell off. Magnets might be okay if you never
touched them, but I made some rings and necklaces that just didn't hold up. Scan through this thread for some really good glue suggestions. I use E-6000.