Avatars are the images displayed next to the poster names, at the top left of the posts themselves.

To create an avatar yourself, find an image you like on your hard disk or the internet, and then go to 'Profile' when you are logged in.

There is a section at the bottom for uploading your image.

At the present time, images must be 80x80 pixels or less, and no more than 10k in size, so if you find an image you want to use but it's too large, you will have to resize it.