Art cover for folders

I have a very large library of music, assembled over many years. I use iTunes Match to get my music to my various devices, in particular my iPhone, which I use to choose music to play on my homepod. Siri is hopeless for classical music. Here’s my problem. My music is sorted into a range of folders, by categories – jazz, classical, rock etc. Within these folders, I have my playlists. Now it’s easy enough to add cover art to the playlists. But how do I add cover art to the main folders. At the moment I have about a dozen identical reddish/purple folders, each with a different label obviously. It would be nice if they each had a different cover, but for the life of me I cannot find a way of doing this.