The supplies I used (magnets not pictured)
Place your glass marble over an image you like, outline it with your pen and then cut out the image.
A few images I chose.
Glue your image to the flat side of the marble, facing up. I like to press down on the image to evenly distribute the glue. Wait a few minutes to dry.
The last step is to glue the magnet to the back of your image. Wait to dry.
And tada! You're finished.
You can print your own images but I've had the best luck with glossy paper. Printer paper seems to absorb the glue and it won't look as neat.