Why would a tenth grade student in English as a Second Language class want a picture of me? He actually took two pictures of me. Do you think I should trick him into giving me the camera and erase the photo from his camara? Can you answer me back? I don't even know him so well although he sits in front of me. I don't know him that much.
Neither you or I can guess why a student wanted your picture. It can be as simple as that he was telling his girlfriend about something you said and wanted to have picture to go along with it. Or, he saw something that he thought was funny that you were involved in and took the pictures. We could guess for the next week and come up with many possibilities and still be wrong. The only way to know is to simply ask him in a non-threatening way. Then he will likely tell you the truth.
"Tricking" is just another way of saying "lying" and Christians should not lie. If you would rather that he didn't have your picture, just say so. But the reality is that you cannot stop someone from taking your picture these days (ask all the movie stars about it). It is an annoyance you have to put up with.