Dude,
Rest your mind. What you have to do, is tell her about your secret, but only in the following way:
Talk about sex, bring it up. Remember, she doesn't know that you know she knows. :-)
Let on that you have an anal fetish. Nothing wrong with that.
If you get to talking about her G-spot, probe her a little on it. I think the female G-spot is very much like the male "G-spot", and you can drop it in that you think so too.
The few girls I've told, including my wife, are really curious. They're like, "Really!?" and the want to know more. Eventually they"ll say something along the line of, "How do you know so much?"
This is the perfect time to tell her that you play with your butt. Re-iterate that you have an anal fetish, and that it feels so good, it's a shame to neglect a G-spot. You wouldn't want her to neglect her G-spot, and she'll probably think along the same lines.
To your advantage, this is also a time to share that you'd like to experiment with anal sex with her. It'll totally turn her on. You guys could have a lot of fun. all you gotta do is phrase stuff so you sound proud and confident about your secret, and then let her know that it's just masturbation and you don't do it with other people. Then, let her know that you would like to include her, and that you are not trying to exclude her from anything, and then leave it up to her. My wife isn't really into it, but one of these days the mood will be right. Now that she knows what I do when I'm alone sometimes, it's not a big deal. I mean, girls have their "alone" time themselves too.
Since she's 18, it's especially important that you are completely honest with her, and also confident. If you sound all ashamed about it, she's gonna think you're some old weird guy. If you come across confident about it, she won't care. At least not after it sinks in.
good luck. oh, and freak out on the person with the photo. be like, who the hell did you get that from? Tell them you were had. That your mates played a practical joke on you. It's not like the photo is a picture of you with the dildo inside you, it's just slightly protruding from beneath the towel.