An avatar is a small graphic or photo that you can select to display underneath your username in each post you make. Here's how to add one, or edit the one you currently have.
First click on the 'User CP' link up in the nav-bar (right under the banner). On the left side of the page you'll see a list of links. Select 'Edit Avatar'. On the 'Edit Avatar' page, select the option that reads 'Use Custom Avatar'. Make sure that the graphic or photo you are going to use is sized at 100 x 150 pixels or smaller. Any photo editing program will be able to resize photos. If you don't have a photo editing program, there are free options available, like
Irfanview.
You will see options to either link your avatar from another site (Photo Bucket, etc), or you can upload an image directly from your computer. Either type in the url for the image, or select browse to find the image on your computer for upload. Then click on 'Save Changes', and you're done!