First, browse the gallery of new icons below, and find one you like, then click on it. After the picture has loaded, right click it, and choose "Save As" (or "Save Image As" if you're using Firefox), and then save the picture to your desktop.
Next, find one of your comments and click on your name. This should take you to your Profile page. After signing in with your password, click on "Edit picture," then hit the "Browse" button. Navigate to the desktop (or wherever you saved the image from earlier), select the image, and click "Open."
Finally, hit the "Upload" button to upload your new icon to your profile. Now all of your comments will be accompanied by the image you've selected, instead of the mystery man in the gray box!