I'm going to take a photo of a girl's butt
Why would you take a photo of a girl's butt?
She's my friend and I like her butt. I'm going to post it on social media.
You shouldn't take pictures of someone, especially of their butt, without their permission. The pictures may end up all over the internet and embarrass her.
She doesn't have anything to be embarrassed of, I'll do what I want with my photos.
She does if you're taking pictures of her private areas without permission. If she finds out about it she will think you are a creep and stop being your friend.