I am having the same problem with some of my album covers. I use Tag&Rename and have found that if the folder.jpg image is not copied to each song, low res images are shown. When the artwork is added to each song, the quality of the image shown improves.
I am also having an issue that some of the artwork is not appearing at all. To resolve this, I find and use artwork that is the same vertical and horizontal pixel size, e.g. 500x500, or 1200x1200. When I do this, pictures appear on the albums.
I have noticed that if the two sizes are very close, percentage wise, they will display, so this doesn’t have to be an exact match, but should be as close as possible.