Personally I always prefer light skins, I find them much more user-friendly and easy on the eye. That doesn't mean that a light skin is appropriate for your audience, however. If your forum is targeting more creative types they may prefer a dark skin.
The type of forum you are considering will include a lot of examples of people's photography and art work. Will that work look better against a dark or a light background? I suspect it would look better against a light background, hence the reason sites like Etsy, eBay and Amazon have those light backgrounds.
For an art and photography forum, my suggestion would be to have a light foreground, which will make it easier for visitors to read etc. But have a nice darker background, which has some great colour's mixed into it. You can find some really nice wallpapers for art which have some amazing use of colour's and you can use them as your forums background (if you get permission to do so of course).
