I did two characters for this instructable: Spider man and damsel...any number of characters can be used, of course, depending on your choice of favorites. This was really easy to do, just need to pick a rock or two, draw the characters on with marker or pencil, and paint! See my previous instructable of painting animals on rocks here:
Step 1: Seclect The Rock, Draw Figure, Paint
I've shown rock selected, reference picture, drawn figure, and painted Spiderman.