It's called an avatar, tereyn. There's information on them in the "Questions About The IRC Message Boards" section of the main message board page. You have to make a certain amount of posts before you can register an avatar.
Avatars need to be either a GIF or a JPG. They must be 65 x 65 pixels or smaller. You can upload an avatar from your edit options menu. To get there click on the "user cp" button at the top of the page.
Perhaps I missed something, but is the moniker "Rouge Improvisor" intentional or a mistype? I love the idea of a "lady of the night" out on a street corner asking for suggestions...