The picture is called an avatar. It is part of your forum profile. At the top of the forum page is a horizontal list of tabs in blue (home, forum, buy&sell, user reviews, etc.). View the drop down under "forum". The pull down menu is in three sections. The bottom section starts with the Settings option. Select settings. Towards the bottom of the top section of data is a row titled "My avatar". There is an upload button. This upload works the same as the post or edit functions within a thread.

One caution, an avatar must be very small. My SSEpic is 7 kb. The original pic is 118 kb.