The most important ingredient to a great looking marble magnet is the glue.  The best glues are clear and silicone based which dry clear.  Next is the marble itself.  You want to make sure that there aren't too many bubbles and don't have chips or scratches.  Also, I prefer marbles that don't have luster and are more clear without any glare, but sometimes the suppliers in your neighborhood may only carry the ones that aren't any good.  It happens occasionally.  With that said, once you have the glue, marble, and magnet all that's left is the art of your choice.  Then basically, all you need to do to make the marble magnet is sandwich your chosen art between the glass and the magnet with the glue holding it all together.