Go to the top of the screen and click 'settings'
On the next page that comes up look at the options on the left hand side and click 'edit profile picture'
Follow the instructions on that page.

This will put a picture within your profile page if someone wants to have a look at your details. If you want to put a picture in that appears in your posts then click 'edit avatar' instead of 'edit profile picture' at the appropriate time.