You could just provide a canvas to draw on and share images instead of strings of characters. If you restrict it to just black and white, and crop to only the used area, it would probably compress reasonably well, but then you'd also have to deal with the fact that some people have awful handwriting, writing with a mouse is hard (I'm particularly awful at it, being a left-handed person who uses a mouse with my right hand. If I need to draw into a computer, I have to get my drawing tablet out of a drawer), you can't paste into google translate for people writing in foreign languages etc