It's probably not the pic that makes you feel guilty
it's the fantasy/thoughts you have while looking at it. Either way, remember that fantasies and thoughts are private matters that no one else knows about and because their in your head, they can't hurt anyone. As long as you don't act on them, you're okay here. If they bother you alot, either find different pics, or have different thoughts. You can change both, you know.
There's no instruction manual on puberty (or masturbation, for that matter), it's on the job training. This is strange because there are so many rules and regulations on just about everything that shouldn't be so hard to figure out on your own and certainly less complicated than these issues! Maybe because there's no instruction manual on these things, a page there to say 'this is normal', people feel badly about them, eventhough they're (usually) perfectly harmless.