I used to be extremely shy about asking for pictures. I still kind of am; like if I don't know the anime, I'm a little more apprehensive about asking. I try to put a positive spin on it: "Hi. What anime are you cosplaying? Oh really? What's his/her name? That's really cool, I like the costume. Can I get your picture?" I did it a few times, but I'm still pretty shy and I always feel like I'm interrupting people when I ask them for pictures, haha. ^_^;
Granted, I was able to get over my shyness to ask for my picture with some cute Chi/Haruhi cosplayers.. heehee..
It's funny though..
First year of kumoricon (2004): 2 pictures. One of vendor's room, and then one of my friend from IRC that had a booth called "Superlime Industries." I was sooooooooo nervous to ask for pictures! Which is really sad, because I saw a ton of really awesome Chi cosplayers that year! I seriously thought if I asked someone for their picture they'd be like, "Omg, pervert! AIIIIIIIIIIIE!" and run away screaming. ^_^; (Granted, at the time I looked like someone out of a death metal band... hehehe.)
Second year of kumoricon (2005): 24 pictures.
Third year of kumoricon (2006): 97 pictures.
Fourth year of kumoricon (2007): 144 pictures.
So, I'd say I'm getting better. ^_^;
(The thing about shyness is you'll never stop being shy unless you stop being shy!!)