Just to let you know, I did a little more digging, and the reason the space was there is that your default Gravatar – which is displayed when the author hasn’t set up a custom one – is the “blank” option, under Settings > Discussion:
https://connectwithtods.wordpress.com/wp-admin/options-discussion.php
If you ever decide to choose another default Gravatar – or ask your authors to set up a custom one, that will be displayed instead, and you could then remove the CSS I gave you above to hide it.
Folks can always set up a free Gravatar here: http://en.gravatar.com/