We definitely want to share questions between courses, so we're going to have to decide how best to do this, given that many of our questions use images. Probably we will set up one course that holds all the quiz questions in published categories (and nothing else) and use the hack from the link above to remove the login requirement for viewing files in that course. Then other 'real' courses would be able to use those questions without the image errors. That would also solve the problem of maintaining multiple copies of quiz questions. It will require a bit more planning so multiple instructors editing quiz questions don't delete each other's work or create an unorganized mess of questions.
Best,
Sue