I think themes are a good idea, as some might be inclined to take very similar pictures each month, not as much variety, ithout the challenge of having to think outside the box. Though I think they should be more imaginative than just matching the holiday of the month, as this would mean the same themes come around every year. (and not everyone celebrates these holidays
) Themes like the ones Gis suggested would yeild much more artistic results, IMO.
I think Karim has a good point about the pictures not needing to be anonymous. It's true that many people have a very distinctive style, and especially after several months, we will all recognize each other's work, anyway.