Here is an interesting article on cognitive principles of aesthetics. Do a search of popular photos on Flickr, perhaps using a neat tool like Flickr Leech, and you will find lots of examples of this technique.
Creating Passionate Users: Cognitive Seduction and the “peekaboo” law
Brains are turned on by puzzles. Brains are turned on by figuring things out. Brains are turned on by even the smallest “aha” moments. And despite what some of you (*cough* men *cough*) might believe, the brain is more turned on by seeing just the arms of a naked woman behind a shower curtain than it is by seeing all of her. So if you’re trying to engage someone’s brain, don’t show everything. Let their brain connect the dots.