Thanks but that's not what my problem is. The avatar appears just fine, that's not my problem. My problem is that, when the default avatar appears, there is, according to WordPress.org, supposed to be an "avatar-default" class assigned to the default avatar image, but that class is not being assigned to any default avatar images.
Maybe if I clarify my reasoning, it will help. I'm going to hide the avatar image completely if it's a default image using display: none; and was planning on applying that style to the "avatar-default" class so that whenever a default avatar image is returned from the get_avatar() function, the image is simply hidden instead of actually displaying a default image.
Just in case anyone was going to suggest it, I did consider using a transparent gif for the default image, but I don't want the comment text to wrap around the transparent gif as if there was an image there.
So, in summary, if I can get this done another way, that'd be just fine (but no plugins because I'm doing this for a theme design):
All I need is to be able to hide the avatar image completely if the commentator does not have a Gravatar.
Thanks for your response anyway deuced.