I think it would be a great idea to introduce a daily / weekly contest where users can submit their creations. You could announce a new subject each day / week (eg. Dragon, Cyborg, etc.) and allow everyone to submit entries. Winners and runner-ups could be awarded with badges.
Not only would this keep people coming back, it is also great for providing users with inspiration and they play around with different tools in an attempt to get their image looking perfect, learning more about Firefly in the process.