I'm looking at it from the standpoint of the pic itself -- you say it's "cheeky," and I'm gathering that your impression is that this is the kind of pic that a girl would not send to another girl for fun, but to another guy.
The elephant in the room for me are the questions that this pic raises: who'd she send it to, and why? What's key to me is what exactly she said to explain it. A reasonable girl would anticipate your questions if she was worth a **** at all and would volunteer to clear it up with you. Something like, "Oops, sry, that was meant for my friend Jessica; she's putting together a collage," or something along this lines is what you'd expect, right?
Her explanation, from what you've spelled out so far, answers none of the questions that her sending the pic raises. Therefore, it's something meant to be kept hidden, and you know what that means.
I think you have more of a relationship with her based on time spent together to say she's ****-testing you. That would normally come earlier in the game.
She's a plate, all right. I would NOT **** a girl like this without protection. And that, to me, spells out exactly where you're at.