Recently there has been an epidemic of users who pull release cover images from Google Images, Amazon or other such websites and add them to MB with no notes explaining where they came from. Many users don't think anything of it (myself once included), but it becomes a problem when that user selects the wrong release (e.g. https://musicbrainz.org/edit/32493819) or adds a blatantly wrong image (e.g. in https://beta.musicbrainz.org/edit/30739200, where the image is a known fan-made Photoshop).
Having editors provide the source of the images they upload will help us sniff out such cases and rectify the situations as they arise.