I'm embarrassed that I masturbated to a picture of my aunt
Why would you do such a thing?
No reason, I just got a boner and I was horny.
Well, you can't control who you find attractive. Just make sure you don't actually act on those feelings, as even hinting at incestuous relations is not okay.
I'm not thinking of having sex with her. She comes to visit on occasion and she's always braless.
It sounds like this was a one-time thing, and you haven't acted on it, even in private ever since.  I would say that there's nothing wrong with finding an aunt physically attractive, but I would have to reiterate that you not mention this to anyone in your family.  It would only be awkward, and wouldn't impress your aunt in any way.