Improve image thumbnailing and increase the size of Macro thumbnails

Summary:
Alternate proposal for D3635.

• Works better with small images.
• Produces a predictable thumbnail size.
• Somewhat reasonable output on 3000x10 images.
• Increase the size of Macro thumbnails to 240px.

Test Plan: {F20497}

Reviewers: vrana, chad

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3638