R1:f763c314e107

Fix external blog images

Summary: Reading the code, this seems correct, but I don't have a local test. Ref T9897

Test Plan: read carefully

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9897

Differential Revision: https://secure.phabricator.com/D16148