Art ladies were never easy, not even before the internet.
You are describing a really strange situation that has very little to od with the internet and everything that has to do with being a human being in western civilization now. The technology is running way faster then the law, and that's running even faster then the codes of human behavior which leaves a lot of us confused about how to act in different social situations.
I'm mostly worried about you keeping a photo that you don't want to keep just because you don't want to be a hypocrite, that's actually a very hypocrite thing to do in my book - yes, I'm all for lack of censorship and equal opportunities, but not in the name of giving up on personal freedom and self preferences, especially when it comes to personal issues, if you don't find the photo attractive, you should have the right to dispose of it, being true to yourself is, to me, more important then being politically correct.
You know what - you might hurt her feelings, shit happens, people get hurt, this is again not because of the Internet but because of not wanting to hurt people, the off line equivalent would be, getting a gift you really hate from someone - we all know the rule to this, say thank you very much and throw it out or give it to charity a couple of days later...
Or - if she's a good friend or someone you are interested in having an honest relationship with - tell the truth. How about "I really like you, but I don't like the photo you have in your profile, so I would love to keep in touch but not through that website", or how about "You are nice but I find the picture offensive and I don't want to et in trouble with the law for looking at naked children, do me a favor and switch it to something else".
I think that the main problem here is not the internet but the fact that you discovered that you are less tolerant to certain type of images then you would like to perceive yourself to be. Nothing is wrong with that, being effected by kiddy porn is actually a healthy thing.