As every relationship progresses, it goes through that stage where the guy asks the girl "What are you wearing?" and goes on towards "Send me a pic" in no time. The "pic" then quickly goes on to become a nude selfie and then it becomes something like a ritual. And although there may be nothing harmful in it for you, it possesses a lot of potent threat to the one who is sending such a picture.
Here is why you should ideally refrain yourself from asking for such a picture.