I moved this over to the Site Help & Feedback section.
Jane Ann (XLR8) is so much better at describing this than I am, but I'll give it a shot...
This thread talks about changing your avatar, which is the little image by your name on every post.
There is also a signature picture, which is different from the avatar. It appears usually only once per thread, and only if you click the "show your signature" box when posting a reply. For that, just follow the instructions for the "Edit Avatar" but instead substitute "Signature" for "avatar" in the instructions.
Finally, there is a profile photo, which is a feature that is available to Supporting Members. Click here for info on how to become one!