The CSS for the border is set by the author-profile-image class. I checked the Github and found that class referenced in screen.css.
The static-avatar class in the same CSS file has a -6px margin on the left side, and looking at the post-card.hbs in the /partials directory I see that the profile images are using that class. Try eliminating the margin and see if that does the trick? I’m not familiar with Casper so this is my quick, five minute look under the hood.