Must be a million things. How about an undersea picture of a wreck ? Or the majestic underground caverns somewhere unspecified ? Or maybe a space picture with perhaps nebula stretching out ? Or go zany and mix subjects like a tree growing in a light bulb ? Or a representation of an emotion such as frustration with a mix of smaller picture blended together representing what's going on inside the individuals mind ? Or maybe your granddad popping out of a large cake in his birthday suit ? I just think you have to sit down and note every weird idea that comes into your head for, say, 30 minutes, then later go back to the list and amply some criteria to boil it down to one idea, or maybe a mix of them.