This is a bit different than other suggestions here but this is my idea.
What if we had an event to add content to the site. Pick one of our top most voted question/answers and hold an event to add new images to it. For example this question: What kind of photography is still better done with film cameras? We could hold a contest to take infrared photos that would be added to the very highly voted up answer on the same topic.
The benefits here are that the contest could be held on any day, and it is adding content to the site which really goes along with the goals of stack exchange. It also rewards the best photo by being presented along with a very highly regarded answer. The downside that I see is if we are editing existing answers and adding photos, I'm not sure if that is advised completely. The community would have to weigh in on that part. It is my understanding since this is a community edited community wiki site, that adding an image to a great answer with no image would benefit all and be a great idea.
