Your Gravatar is an image that follows you from site to site appearing beside your name when you do things like comment or post on a blog. We've switched them on here at Newsbiscuit, so you can have a little picture beside your name.
The process of uploading your picture happens entirely through Gravatar - you need to use the same email address as the one you registered with here. Follow their instructions, the whole thing takes less than 2 minutes. Then when you come back here, there will be the same picture beside your posts.