Prefer PNG for thumbnails of PNG

Summary:
I am not sure if anybody but me can see it but JPG used for non-photo images is extremely ugly:

{F9506, size=full}
{F9507, size=full}

Test Plan: https://secure.phabricator.com/file/xform/thumb-160x120/ of PNG file.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2039