Avatars

Hi, I’m kind of a novice when it comes to WordPress, and I would like for people who comment on my blog to be able to use avatars. I installed a plug-in, and the menu showed up in my admin pages when I activated it, but when I upload images to use as an avatar, all I get is a red X. What could be causing this, and how can I get this feature to work?

Also, what is the difference between and avatar and a gravatar?