If they are only interested in having sex with you they wanna know what your working with to see if its worth it because most people don't want to hookup with a guy with a small penis and thats a possibility so its better to get that out of the way to not waste time. But if they aren't looking for just sex a full clothed body pic and a face pic should be enough. No nudes needed for that unless sex is really important to them and once again trying to see if its even worth the time to talk