Yes, it certainly can. You have to be careful though which plugin options you select and how you cache your content. The first plugin option checks if a gravatar exists for a given e-mail address and, if not, displays the default image directly. If you cache the output of this check, the gravatar will not be displayed if the user later decides to sign up with Gravatar.com. If you’re using this option, you should invalidate cached pages with gravatars on them (mostly the single view of entries) regularly.
