Was this illegal on my part?

It was probably illegal on his part, not yours. If there was a pic that he saw it is important because although you thought you looked "mature" from the picture any normal person could tell you were not of age. But most importantly, why tell your mother now? It's part of your dumb kid past. Did you tell her every time you sneaked a drink, told a dirty joke to a girlfriend, soiled your undies? Why would you WANT to talk to your mother about a single stupid cyber conversation, unless there is more to it than you are saying.