We should allow admins to manually input alt-text for item images which overrides the chosen metadata element.
Additionally, it might make sense to create an "alt-text enforcement mode" which requires all new items to have a specialized alt-text field, instead of just choosing a metadata element. Alt text should be rich, and specific to the image. An item description, for example, may not actually describe what the image shows. Sometimes an image might be very different from the description of the item it represents.