Like this "hey, if you don't mind, can i see your picture? it's probably cooler to know who i'm talking too... blah blah".
Asking for pictures on the net is creepy though, there's just isn't a smooth way to do it, if you have to ask for it then you're not gonna get it (most likely). If a girl GIVES you a picture on the net (specifically for you only) then it's wierd too... let's just say that there isn't a good way to do it, if you ask for it and she gives it up then everything is ok, if not just let it go, can't do anything else right?