Increase the maximum size eligible for image transforms configurable from 4MB->16MB

Summary: Also increase the timeout for the external process to complete the transform.

Test Plan: Careful inspection

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: joshuaspence, cburroughs, chad, Korvin

Differential Revision: https://secure.phabricator.com/D14528