Being ruthless, I'd guess there's a solid majority on broadband. And there isn't much that can be done do for those on dial-up because anything bigger than tiny thumbnails is going to be too much for them. (Never mind those using mobiles.)

So I'd be happy with 600 pixels for the maximum dimension, and a size of 50-100Kb.

Re: content - we could maybe have a very loose theme announced ahead of time, so people who have the time and inclination could take some new photos. (Or pick out some favourite older ones.)