<r:gravatar/> returns a url so
<img src="<r:gravatar/>"> would show the gravatar for the author of the current page <img src="<r:gravatar user='bob'/>"> would show bob's gravatar
<r:gravatar/> returns a url so
<img src="<r:gravatar/>"> would show the gravatar for the author of the current page <img src="<r:gravatar user='bob'/>"> would show bob's gravatar