I've been looking at this, and I'm thinking the issue is simply caused by more obscure music not necessarily having results on Deezer, etc.? The sample track (BFG) shows downloaded art for me at least now, so I'm guessing the user who reported it is no longer having the issue?!
However, I've added some code to my fork using the StorageItemThumbnail class to essentially pull the image file from the tag in cases where the online lookup fails. See screenshots.
Does this sound like an okay approach? This appears to be a fringe case, so the existing artwork download mechanism should still be handling almost everything...
The Doom artwork on the left is what was embedded in the provided sample mp3. (Note: I removed the other id3 tag info so that the lookup would fail for the first one.) The other Doom artwork was downloaded.