Make CSS agnostic to underlying profile image size

Summary: Ref T7707. My analysis there was a bit confused and this isn't really all that important, but seems cleaner and desirable to be agnostic to the underlying image size.

Test Plan: Tested Safari, Firefox and Chrome with a variety of profile image sizes.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7707

Differential Revision: https://secure.phabricator.com/D12825