Wow, Mosh, that kinda sucks to be in your position. However, lacking y-chromosomes, you do have a slight advantage - you can always play the sexual harassment card if you even slightly perceived any negative reaction toward you, regardless of what course of action you take.
Thus, copy them to a CD (never, ever do this with illegal pornographic material - regardless of the intent, that makes you just as guilty) as a backup as proof that you felt uncomfortable working in an environment where male coworkers were staring at such pictures on the company computer.
Then, delete the images & say nothing. As the photos are "legal" - there are no legal consequences toward the person who downloaded them in the first place. Since that person is friends with the head honchos in your workplace, then I doubt anything would be done to him, other than a slap on the wrist just for the sake of appearing to be enforcing whatever workplace rules you have.
But, as I'm suggesting, in all likelihood, nothing will ever be said if those photos are just deleted. But, in the unlikely case that something IS said, and it even slightly looks like it's painting you in a bad light, then play the sexual harassment card.
edit: this should have been prefaced with the idea that I'm under the impression the type of work environment you're in, there are probably no more than a total of 25 employees? If we're talking about a significantly larger company, then by all means, turn it in to a female in HR (if there is one.)