Sending nude photos.?

[Mitch]

Well, last week a girl added me and inboxed me on Facebook, we started talking nicely. Then yesterday she sent me an inbox saying he was horny about me. I'm only 14, I was really confused and didn't know what to say, I told her I didn't know her and for all I knew it was my friends pranking me. Obviously it wasn't when she sent me proof, she send me a naked photo of her and it was the exact same as all of her pictures. Her profile is real, trust me on that. She wanted nudes back. I said i'd have to think about it.
Just 5 minutes ago she messaged me asking me If i've thought about it. I really don't know what to say. Please advice anybody, what I should do, and say.
Okay then, I won't do it. How do I tell her no?