I’ve had gravatars on this site a few times over the last few years, but they always seem to disappear whenever I do a retheming. I’ve added them back in the comments and also added the last comment to the main page along with a gravatar.
If you don’t have a photo next to your name when you comment, that means you don’t have a public gravatar available. If you want one, head over to Gravatar.com and sign up. It’s free, and as soon as you add your photo to it you will start noticing it automatically show up on various sites around the internet. If you don’t define one you’re going to end up as that stupid soccer ball from Windows on this site. And who wants to be the stupid Windows ball?
Feel free to drop a comment to test out if your Gravatar is working. If you’ve recently added one, it may not show up for a few minutes due to caching, but be patient and eventually it should.