In my case, I usually spend months before the competition filling up on cool ideas I am going to get around to writing about someday when I have time. Sometimes I even jot them down in a list so I won't forget them.
Then, when the pictures go up, I stare at the pictures and try to figure out how each of the things in the pictures arrived at that moment in time--what would have to have come before, what is likely to happen after. Since use as a static image is all but prohibited, it's important to think about everything in the shot as an object in motion, undergoing endless change, in some context. Folowing Piratecat's lead, it's interesting to look for ways to understand the motion that aren't obvious.
Also, I'll make lists of the obejcts in isolation from thier contexts and think about what symbols or metaphors fit these things.
And then I'll try to think up some characters suggested by all of the above, and what each one has in his or her pockets. That's a metaphor. Sometimes the inventory is not physical objects--but the idea is, what does this person walk into this story with at their disposal that no one else has?
Then I try to connect the dots.
Usually I have way too many dots to coonect by this point, so it's largely a matter of spilling out a huge heap of ideas and then carving it down to just the good stuff.
When I get stuck, I take a break and do something utterly unrelated, and then I go look at the personal inventories again. Getting unstuck almost always comes from getting to know the characters better. I have to stand in their shoes and pretend I'm a PC: the GM just handed me this sucky situation, and here's what I've got at hand to respond with--a list of personality quirks, skills, experiences, objects at hand. If I can't get the answer through one set of eyes, I move my point of view to someone else for a while.
Characters make the story happen, and plot connects the dots so I really focus there when I'm writing. The metaphors and preconceved notions of what would be cool to write about just percolate in wherever they happen to fit. They are things in my inventory as the writer, and most of them stay in my pockets, unless there's some really good fit. A lot of times, I can tease myself with getting to use one as a carrot for getting a scene out. Usually I find it tastes different than I thought it would once I've prepared it. But when one starts really working, it adds a depth to "what this story is all about" that is intoxicating to me.
That, and I drink a lot of caffeine and Southern Comfort or vodka to get my brain limber, which is another reason I don't write when I'm pregnant.
