I think you are on the right track with getting ideas early. It seems, with event 4, like there is a bit of trying to get the ball rolling by comittee. Of course, a handful of people contribute, but each has their own expectations, limits, and busy lives. As ideas get bounced around, some loose interest and move on while others may just give up. For some, too, I think the willingness to participate is both timing and implulse, which may not last long if Given too much time to think about it. I can’t help thinking that it would be better to just come up with a list of photo requirements, and post with a call for entries and a deadline. But there also has to be a fairly regular, and somewhat more active, pool people on the site to see it. This means engaging and maintaining interest in the site, in general, and possibly some email marketing strategies?
The other apparent problem, which is very much ties to getting participants, is engaging non-participants and getting them to vote, of course, and also input ideas for what would they like to see (that would get them on board?), and to talk about it on the forum.
Getting stripped by, and for, a larger audience that includes both women and men would probably be a plus for many of the participants.