The entry created by "remote user" should have either some generic or no avatar.
Entry for "remote user" links to non-existing avatar image, showing ugly placeholder instead.
js/activitytabview.js:170 element.avatar(element.data('user'), 28) generates a broken link if user is empty.
The main activity view js/script.js:254 uses the (first character of) the display name as "icon" if a username is not available. In the case of "remote user" (hard coded in lib/Formatter/UserFormatter.php:54), that's the quotation mark. Whether that's the best option is open to discussion, however it's definitely better than the broken image. I'll submit a pull request to do the same for the tab view shortly.