I like RedPhantom's ideas.   There isn't really a need for prizes if this is to become a monthly event.   I actually prefer monthly to semi-monthly;  gives those of us with work, family and other priorities a chance to actually do something.   Two weeks usually translates into just 2 days for me :)

The black and white idea sounds like it would be good just like other colour challenges would too, though spaced apart if some ideas are too similar.   Have a look at the RDNA challenges, winners get recognition in the form of an icon in their avatar or something and if you look through the challenge gallery you'll see most themes were general enough to allow for more creative involvement.   If it were  too constraining then fewer people would participate IMO.   I'm always amazed at all the great art in those challenges so RDNA must be doing something right. 

I'd also like to see the images in a separate gallery not in a forum thread.   I think the Contest gallery would be best for this.   You can easily see what the current challenge is, what previous ones were (with the contest archive section) and voting can take place there and not have to be announced in a thread or through "clandestine" emails :)