On my old phone I literally had a ton of nude pics from various girls. Some were just online (hey, if they're hott and half a country away mine as well get something out of it..) but a lot of them were girls I know and/or have hooked up. You basically just have to get them really hott. Just casually lead the conversation into something sexual through text. Here's how a lot of mine usually go:
Scars: Hey girl, it's been awhile. I miss you.
HB: Awh, I miss you too!
Scars: I'm so bored right now.. I wish you were hear with me. I would kiss you all over.
HB: Me too. Ohhh, really now?
Scars: Yep.
HB: I'd really like that.. I wish you were hear right now.
Scars: Oh ya? What would you do to me if I was right there next to you?
HB: Oh, idk. Kiss and stuff. What about you?
Scars: (Then from here I would type something really descriptive that would get her hott.) I'd press my lips against yours softly, gently brushing my tounge against yours as I run my hand along your cheek as my lips travel from your lips down to your neck.. (sometimes it initiates text sex.. if the girl is freaky enough, but often times the girl will just reply with something like "wow.. id really like that" you could ask and be like "then what would you do?" if she doesnt, then dont force her.. often times she'll just be like "mmm you keep going" and just type a bunch of **** to get her real hott.. but escalate this ****... then half way into it be like "I'm so horny right now, I'd love to see a picture of you.." often times she'll ask you what want to see.. just simply tell her. She'll probably be honry and wont give a ****.. I've gotten so many pics this way. Or if you have this girl on lock down just simply ask her.. if she likes you enough, she will do it. But I think this way it's a lot more fun.. I actually was texting a girl getting her hott once.. then she was like "I can't take this anymore.. come over right now." so I came over and ****ed her brains out.
Good luck man.