"Difficult to read" report button on all images.
Blurry scans of original documents are very frustrating. I sometimes use the Feedback sidebar to report them but what might be more useful is a separate sidebar button labeled "Blurry" or something. It might not need to collect any details from the user, just record counts. It could display the accumulated count. It could also present a multiple choice menu of image quality issues: blurry, page rotation, catalog error, etc.
I think this would be a useful outlet for users' frustration and over time could generate a priority list of films or original documents to scan again.