By all means, have a go at this. It will be exciting to see how it could work.
That said, it's really not the role of the forum admins or moderators to host such a contest. It does not fall within the mission and purpose of this forum, which is to maintain an open forum for dialog and the exchange of knowledge—etc., etc., etc., blah, blah, blah—there are more reasons but I'll stop there. The ONLY thing I mean by this is that we would not start, host, or run it as admins or mods. We will not prevent a user or users from proceeding with it using current forum functionality (though the forum software is really not set up to implement contests or voting in some of the ways being discussed here).
So, one thing that might work to organize the images is to ask folks to post images to their own galleries with the images titled with a specific hashtag, so that one could search the galleries for that hashtag and see all of the images together.
The other option would be to simply ask users to add images to posts within the topic for that particular challenge (keeping in mind that you cannot limit posts in a given topic to only those you want to see in there).
The polling function in a topic has an option for up to 20 answer choices, and the topic poster would need to populate those, but that means only 20 images could be considered for voting. So, it's possible someone could launch the project (start a topic), and tell people to post images using either method described above, and then open the voting through the topic polling function, but it would be limited to 20 "finalists" or 20 participants to begin with.
I suggest someone try it and see how it goes.