How to fix mismatched album artwork after iTunes sync on Mac and iPhone?

I did a recent sync of my Mac and iPhone. Most of my library is imported from my CD collection. Only a handful are downloads from iTunes. What happened was twofold: songs were duplicated in most of my library, and album artwork, which I painstakingly downloaded for recordings that iTunes couldn’t find, was finally added to my library on the phone.

To remedy the duplicate songs, I followed the prompts on this support thread: Duplicate songs on phone only - Apple Community and deleted all music files from my phone and did a fresh sync. The duplicate songs were eliminated, thankfully, but now album artwork is all over the map. Multiple artists now carry the same album artwork across many albums.

For reference, I created a separate folder only for Album Artwork, “iTunes Album Covers” and placed it in my Music folder. Could this be the culprit? I do have a folder titled Album Artwork inside my Music folder (image attached) which shows folders titled Cache, Cloud, Custom, Download, etc. 

Any suggestions on where to start? 

[Re-Titled by Moderator]