I've always used, "Send me a sexy picture.
"
Of course, I wait until we are in a heightened peak of conversation and things are going well to ask it.
It's interesting some of the results you get. Some girls will just send a picture of their face. Some girls will send you a picture of them in their bra and panties. Others will send you topless etc.. it's almost a simple way to test a girls skankness and/or interest level.
If it's just for a phone profile, then what you said earlier is fine. But if you're looking for a PICTURE PICTURE, then make sure you wait until her emotional and interest level for you is high. Even if it's only for the moment. Butter her up a bit. Be playful, and tease her, then just ask. Or, if you want to go the other route, just simply hint at it through some innuendos. But there is going to be a point where she is either going to ask you straight up, or you're going to have to make it clear exactly what you want. It's part of being a man.
-Scars