It does sometimes happen but I agree with IAN2411 that you should be very careful. Anyone putting pressure on you to send pictures of your body should be distrusted. If you have gotten to know this person and you feel that you would like to send pictures, there is some things you can do to make it slightly safer, but it is still a risk.

* NEVER, EVER, EVER, ever send a picture of your face and any naked body part.
* Photoshop out any distinguishing marks that you might have on the photographed body part.

Something people often don't think of is that the email address they use(also to send pictures), often is their vanilla email address. It is a small step from your real name to Facebook to knowing much more about you than you feel comfortable with.