The key is convincing her that not only do you need to see her and want to see her, but that she is so hot that it’s a crime for you to not be able to view her best assets 24/7 and at your very fingertips. No one said you have to believe it, I’m just saying you have to sell it.
No need to go for full nud*ty at first, you can definitely work up to that. It’s best to start simple. What’s your favorite part of her body? Tits? The way they look in that particular bra? Her ass in those hot boy shorts? Tell her what it is you like about them (name the particular bra or other undergarment of your choice) and why that’s what you want to see. Once she gets comfortable with that, and trusts that the pictures are being kept confidential, you can encourage her to get more adventurous.
Girls like to be wanted and appreciated, and if you tell her you’d love to take or receive her picture, because she’s just too good not to be freeze framed, if you do your job right, she’ll believe you. She’ll think she’s special, and that you feel this so strongly, just about her, that she won’t say no (why do you think artsy types are so good at getting chicks nekked?).
Most importantly, as Flow also pointed out, be appreciative. When that little picture symbol pops up in your inbox, don’t forget to tell her how incredibly sexy she is, and you’ll have a personal spank bank before you know.