Ravatars will generate and assign random icons to the visitors leaving comments at your site. It can optionally show Gravatars as well.
To change the source images, replace the files within wp-content/plugins/ravatars/parts with your own images. Supported image types are .jpg and .png. Its hard to say how many images you should upload. The minimum for a good set of ravatars is probably around 40, but you will want more if you have a lot of people posting and you want avatars to be unique for everybody. Try to get images with a lot of distinctive elements in them.
If you are using WordPress 2.5+ you need to have an avatar enabled theme, and ravatars will automatically show up.
If you are using WordPress pre 2.5, read on for tips on how to position ravatars on your site.
The administration panel is under Options » Ravatars. You can adjust the size of the Ravatars, and assign HTML to come before and after each image to help nudge it into place. Each image is also set with the CSS "ravatars" class, so you can fine-tune the avatar position in your CSS file.
If that still doesn't give you enough control over ravatar placement and you don't mind editing your theme, just turn off automatic
placement and add the line
ravatar_show($comment->comment_author_email); to your comment loop wherever you want the image to appear.
Your mileage may vary. How it will look depends largely on your current theme.
Note that the plugin requires that your install of PHP support the GD library. If it doesn't, the Ravatars won't show up and you'll get a warning in the Ravatar admin panel. You can still use this plugin to display Gravatars, even if the GD library isn't available.