To your profile or an avatar picture, need 100 posts to add an avatar picture. Either one from message screen, control panel on left, click on choice, save once you have chosen picture from your computer.
This was a writing exercise, a caper we thought, we created an imaginary persona to be sassy with men who PM ladies in a rude manner. Eventually ladies started to PM also and I met some nice ones. This went on for over a year and it stops now.