An avatar is a small image that can appear on the left hand side of your posts underneath your username. (Please note that the system will only allow images up to 110x110 pixels or 390.6 KB, whichever is smaller)
To get an avatar, simply click Forum Actions (found at the top center in the purple bar), then select Edit Avatar under My Settings located in the left hand column on the page. You can then choose to either upload an image from your favorite photo sharing site or upload it directly from your computer. Be sure to click save and you're all set!
*Note* You will want to additionally upload a profile pic that will be appear on your personal member page/wall, show up in comments on articles, blog posts, on friends' walls and more!