You might want to think twice about doing a calendar.
People will get miffed if their images don't get chosen for one of the months.
I guess the only way to alleviate that is to conduct a vote. Assuming, of course, that someone/people is/are up to the task of finding the images in the first place to be voted upon.
I can't complain, though; two of mine got chosen for the Canberra Astronomical Society 18 month calendar, lol. Yay, another publication!
Regards,
Humayun