hmmm, in this case though its a full image that is not theirs. When a person does that they have no idea what terms of the model release they might be violating, the avatar represents a person and their beliefs, or any use of the representation of the image, by extension that image comes to represent those same beliefs or at the very least is associated with it. Those might not be in line with the artist's or the images intended representation. And so on.  While its not really worth the heavy hand of god to correct it, it is worth a gentle request to stop using it.

cheers,
mark