A WP image url links to Facebook as Gravatar.com url

Have you installed “Share and Follow” in your site? That plugin by default uses your gravatar as the default post image when sharing. If you are, then go to the Share and Follow settings page and find “Setup share image”. Click option “no”. You won’t be able to see the change on old posts, but should on new ones. If not right away then in a couple days. Hope that helps!