My mother in law just sent me a photo of my son dressed in a dress and with his hair tied up for fun. I am quite upset about it but my friends are telling me it's ok. Do you think it is wrong for her to do that?

I think you have a right to be upset - he is your child and it sounds like you feel that your mother in law breached your trust by doing something that you don't feel is appropriate. I would have a conversation with her about your expectations. Personally, I wouldn't mind. Kids play dress up sometimes! It is just a way of experimenting and understanding the world.