Flagging questions in quiz does not work - "flag-on" image missing

The adaptable.css stylesheet refers to a "flag-on" image for use when a quiz question is flagged by the user. The "flag-on" image is missing, and no other appropriate image seems to exist in the repository.

I don't know what the intended image is, but it should probably be added in pix/quiz, and line 2007 of adaptable.css modified to point at it.