As far as crowd-sourcing the pics goes, it would be doable to turn it into a contest of sorts. Just create a couple of threads for 'm and let the forumers vote, I'd think. Polls would work, I don't think Unreal World has an audience so huge there's a real risk of people signing up for dozens of throwaway accounts to votestack without it being rather, eh, noticable in the total numbers.
Maybe one thread (preferably stickied and maybe in the forum index news) to inform people of the upcoming contest, the various scenes and times of day needing pictures, whatever rules are necessary (I'd say that at the least, pictures must be relevant, own work and released under one of the appropriate creative commons licenses to avoid a myriad of potential issues. Probably a limit on number of pictures folks can send in, too. Either one or two per scene-type per time of day, I'd suppose.) due date and how to send in the pictures (I'd think an e-mail account created for just that purpose to avoid hitting the PM inbox limits/various PM shenanigans?) as well as relevant info on how and how long the voting will be.
Then a poll-thread per scene-and-time combination with all the relevant pictures in it, with a set runtime for the poll and links to each of them edited into the info-thread's first post (& repeated as new post in thread for those only checking new posts). Would be nice if there could be a separate subforum somewhere for the sake of the contest to avoid flooding any particular existing subforum, but not strictly necessary. If in an existing subforum, the vote-threads should probably all be pinned to keep some from getting lost in the middle of everything else.
All in all, that would require no more from Sami than to sticky a bunch of threads (at minimum) or to sticky a single thread and create a subforum (which, if Simple Machines is anything like the forum sites I've worked with, shouldn't be overly much work nor very difficult) for the vote-threads.
It would involve a fair bit more work from one or more regular users, but certainly not beyond what is possible with a bit of proper planning (and some real-time communication/coordination through irc-chat or text-skype or w/e) and a few people willing to put in some time and effort.