It would be really helpful it the plugin was updated to reflect the fact that not all testimonials will have associated images. Even if all the testimonial providers used Gravatar, a website owner may not want the maintenance of constantly checking that the displayed avatars are appropriate for the website content.
I’ve tried setting the image size to zero, but %%AVATAR%% is still displayed.
I don’t know about that one, but providing you set this one up correctly it supplies a mechanism for opt-ins, and will drop the cookies only after opt-in.