Go to the link User CP on the menu bar.
Once in the User CP you have at left all the options
To change the profile picture go to "Edit Profile Picture"
To change the "avatar" wich is the image that we see on the threads .. click on "Edit Avatar"
I think that there is already some thread about this, before posting a thread is a good idea to do a thread search, using the search buttom on the menu bat at the top. Your answers comes faster this way