It doesn't have to be 12 months. You can use
lulu.com to create a calender with as many months as you wish, and its free to create them, with a minimum price that can be added to to create a profit. They're pretty high quality from what I understand.
As for what to put, I'd say a vote. Perhaps create one thread strictly for submissions and another for members to place votes for however many will appear in the order they like them. If it doesn't come out perfectly, do a poll to decide the final product. There's probably a simpler way of doing things, but that's my solution.
I also like the idea of both an X-rated calendar and one strictly for member art. I'm sure both would sell well, and heck, you could do a ladies and mens calendar for the X-rated one.