What kind of trouble could you get into?
it's your site, you are incharge so you can do whatever you want really

If you want to have people putting up images at first to get the site going then there is nothing wrong with that, there is no difference between someone posting images and not getting paid and someone getting paid and posting more.
It would be different if it was effecting your income in regards to advertising or something like that, for example if you were paying someone to post the images and you were getting paid by advertisers per page/image that is posted.
That's just IMO though, others will have different ideas.
With your type of site, an image rating site, I don't see any problem with it though
John