I like the idea of a premium gallery. I think it would be a great thing. You could also use it for prizes for future contests: award a certain number of postings to the premium gallery. I would recommend making the cost based on the number of images to post, rather than for a certain length of time. This should reduce the burstiness of posting of images, by removing the temptation to buy a month and "make it count", making it more likely for people's images to stay on the front page for a while.