I just of thought something else and it is probably what I would have done. The only real problem is her guilt. Handle the naked business as being entertaining. Ask her which people look ugly without clothes. Ask her if they wear nice underwear. Be light-hearted. Whenever my children got overly concerned about something, they took their cue from me. If I was really laid back, it eased them. And I did use a lot of humor. If you can turn her imaginings into an amusing quirk, she will not feel strange or abnormal (which she isn't).
It's a difficult age and you are doing just fine. Fortunately she will get over it when her body settles down. Comfort and patience from you is what she needs and what you are giving her.
You may also want to check what she watches on TV sometimes it is triggered by viewing nakedness on TV and the PC , check out what she sees at your house and other peoples houses,
I agree with clairebear3737. When I was younger I found it exciting to picture certain people naked in my head, sometimes other girls. But time straightened things out for me including my friends. Never did pictured my mother naked but pictured my father naked once. Never did see him naked. I belive it is all apart of growing up. At least for me it was. Only time will tell with your daughter. Just keep the lines of comunication open don't cause her to feel guilty about what she thinks. It sounds like she is carrying around her own guilt.
I dealt with the same things at that age, the questioning my sexuality and general sexuality questions. I think its normal in development with puberty hitting and what not. If it bothers her alot though it probablly would be best to let her speak to someone, but as a parent I wouldn't dwell on it with er to much unless she brought it up to you.