But this is, of course, only a wild guess. I didn’t mean to suggest they got upsampled, rather that on my main computer they are cached at a lower resolution – more appropriate for a conventional monitor.
If you have any files in the folders which are empty for me, and can figure out what the artwork is for and where it comes from, post a comment so I can update this article. On my iMac, that folder is 4.46 GB, and on my MacBook, with a much smaller library, it’s just over 2 GB. It’s actually a good thing that Time Machine does not back up the Cache folder, which is the largest that would eat up a lot of backup space. The folders I see in my Time Machine backups are Cloud, Cloud Purchases, Custom, Downloaded, and Remote. Interestingly, only some of these folders are backup up by Time Machine it looks as though Time Machine knows to not back them up, because they are essentially temporary folders, or folders whose content can be easily recreated.
Editorial: This folder holds artist photos that display for some artists when you view their music in Artists view.There is also artwork for audiobooks that I have purchased from Audible. I also see thumbnails for some albums that I have purchased from the iTunes Store, and perhaps for albums where iTunes has downloaded artwork. Some of the artwork is for podcasts or iTunes U courses that I have downloaded, some are four-album graphics as used for playlists. Downloaded: This folder contains a number of different items.Custom: Artwork that you’ve added to a playlist, by dragging it to the album artwork in the info bar above the playlist when in Playlist view.Cloud Purchases: This is artwork for items you have purchased that are still in the cloud, and that you have not downloaded.If you don’t have iCloud Music Library turned on, this folder is empty. iTunes downloads artwork from the cloud so it can display this artwork for these tracks. Cloud: This folder holds a cache of artwork for tracks in your iCloud Music Library, but for which you don’t have local copies.This cache allows iTunes to display artwork more quickly than if it had to extract it from your files each time it is displayed. Cache: This folder contains cached copies of artwork taken from your media files.The only app I have that can display these files is Graphic Convertor.) If you see artwork in any of the folders which are empty for me, have a look and see what they contain. Here is a list of what I’ve figured out so far. Several of the folders are empty, so I’m not sure what they should hold. I’ve looked through the contents of these folders on my two Macs in order to determine what the artwork in each folder is used for. They all contain album artwork, but each subfolder has artwork from a different source, or used for a different purpose. But recently, that folder has added a number of sub-folders. ITunes has long had an Album Artwork folder (in your home folder, in /Music/iTunes), which held a few sub-folders for cached artwork, downloaded artwork, etc.