Give Pholio images a more modern initializer method

Summary: Depends on D19910. Ref T11351. Minor changes to make this behave in a more modern way.

Test Plan:

• Destroyed a mock.
• Lipsum'd a mock.
• Poked around, edited/viewed mocks.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11351

Differential Revision: https://secure.phabricator.com/D19911