The game has a way to submit screenshots to the FEED if turned on.
Which is located here:
http://www.trainzportal.com/mytrainz/
Why not just attract people using the game its self.
Make a popup letting the user know they can submit a screenshot for competition with "insert theme here"... etc..
And you can make checkbox for users that don't want to ever see screenshot competition messages or they can turn that ON...
This prevents screenshot modification outside tane.
Not sure if you can make a new message on the screenshot tab in tane that tells users about the theme of the current week.
REALLY: The "upload to feed" feature is something that people need to know about, ITS SUPER EASY TO MAKE AND find your screenshots already online with the feed, because they have no idea on how to find the screenshots, let alone upload them to imgur and THEN even link to that image in a forum.
Integrate it with the game/feed so people don't even need forum access.
So you already have the feed available for people, just integrate the competition more with the games screen shot interface and too many people will probably participate, overloading your feed servers...
Then you can give error if user submits more than one for current theme,,, etc...