Django integration. Uploads, edits/versions, user accounts (persona), invites, comments.

Multiple image upload.

Sortable photos with saved position. Fiddle. And another implementation. Each version should also save the selected filter setting.

Versioning of edits. Could do it manually with counter? Or use django-reversion? Or django-simple-history?

Persona authentication.

User accounts.

Send email invite to collaborators.

YesNoNeutral in preview Colorbox (use Colorbox's preservation of JavaScript events for inline calls.).

Touch-friendly (drag and drop works on iOS, Android).

Resize thumbnails.

Squiggly grease pencil lines.