I looked into them (along with Corbis and Getty) before deciding to go the microstock route. Among other things, Jupiter requires that you have at least a 12MP camera, which may leave out most of the people here. All three agencies more or less expect you to be able to completely manage a professional photoshoot (e.g. hire models/stylists/assistants, pay all expenses, arrange for travel and accommodations, ...), which is also probably beyond the expertise and experience of this crowd. Bottom line: I don't think they're gonna really consider you unless you are a professional with at least a little experience.
You can, alternatively, develop a portfolio and offer it for sale to them.
A good way in is to know someone: I recently met a guy who was shooting stock for Getty - he got jobs because a friend of his was an editor there. They flew him out to Vancouver from NYC and even supplied him with a Hasselblad and one of
these babies!
Here's the questionnaire Jupiter sends you:
click me. I wasn't very impressed by its rather amateur appearance.
A telling sign: Yuri Arcurs was approached by one of these agencies and decided that he was far better off shooting microstock than shooting for a large RM/RF/Day Rate/Work for Hire agency.