I'm inclined to agree with RJ and Delphinium.
You feel it was an invasion of your privacy to delete your FaceBook post? What about the invasion of hers, that you put her photo on there in the first place?
You think she was treating you like a child, but I would guess she felt you were treating her like one. She would have felt a total lack of control over her situation. She asked you not to put the picture on Facebook, but you did it anyway. (I do appreciate that it was about the timing, and if she'd asked a few seconds earlier, you might have had time to talk about it, first.)
I understand that her obsession with her looks can be frustrating for you, but, rather than being vain, it's more likely to be a symptom of insecurity about her looks.
If photo taking is taking too long, just take some without her, or don't take any. Photos aren't as important as happy memories.
Also, I think quite often children with a "difficult" sibling can have a hard time of it. They are stuck in the "easy child" box, and have to constantly live up to those expectations. Their needs can be overlooked, as less important. You really need to watch this, to ensure that she feels like you see her as a whole, complicated, human being, in her own right.