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Copy the easygravatars folder and its contents to your wp-content/plugins directory, then activate the plugin.

The plugin will add a new Easy Gravatars section under the Options menu. There, you can configure the size and maximum rating of the gravatars that you wish to display. You can also set the location of a default image to display for users who have no gravatar (the default is a 1px transparent gif), you can tweak the CSS for the wrapper around the image, and you can select to attach the gravatar to either the comment author link, or the text of the comment.

By default, the gravatar is floated to the right of the comment author's name, which should work well with most templates.