I agree with anutharound, that stuff works well. I can't figure out why some of the guys on here (a website for picking up girls) are uncomfortable with getting naked pictures of girls.
As for not taking the pictures yourself? What about if you don't live anywhere near them and are unable to take the pictures yourself? As a man stationed overseas where porn is not allowed, my only source of material is from girls back in the US of A. I have been here for about 8 months and so far I have 6 girls regularly sending me naked pictures because I asked them to "support the troops overseas".
Its possible, and its a completely valid question. It is a LOT easier to fclose a girl if she has already sent you naked pictures because you don't have to get through a lot of her usual defensive bullshit.
Like anutharound said, make them feel comfortable, compliment them.
First start out with her sending you regular pictures.
"Oh, you got a hair cut, let me see! Take a picture and send it to me."
If she doesn't have a camera or webcam, have her send it on her phone (if you dont have a phone, tell her to put your email address in the "phone number" field)
This is essential because it establishes the fact that she can indeed send photos. Once you break that barrier, work on some risky ones.
If she gives you an opener like "I just got out of the shower" or "im in my bra" then you should say something along the lines of "You should send me a pic

" (smiley face included)
make it seem like a joke because if it backfires you just say "jk" and move on....but if she says maybe, or not now.... just wait and try again later. If you make it funny, there is a lot greater chance that she will lower her defenses.
Be prepared with an answer for if she is considering it. "No way, you would send it to everyone"..... If you know the girl, you should have decent reasons of your own for comforting her. I always say "Listen babe, I have a beautiful girl sending me pictures and you think I am going to do anything to ruin that! You are crazy! I love it!"
Remember, if she sends you the pictures, always say how good she looks. If you know she doesnt like her eyes or hair or something, always be sure to say "Your hair looks really good"
Try not to just say "Woah, your tits are awesome" comment on something else about her and you will get more pictures in time. Remember, sluts are hot, women are beautiful so be sure to tell her she is beautiful, or she looks amazing or something along those lines. It sounds classier.
Anyway, good luck!