I'm not sure if Jenna Bush would be called a celebrity. I don't even think she's particularly good-looking. However, the law of the internet states that when you find pictures of the President's daughter changing out of her bikini on the beach, you post those damn pictures, and that's all there is to it.
Please note, it's not 100% whether these are real or not.
Update: Your friendly neighbourhood Fleshbot has confirmed that it is, in fact, not Jenna Bush.