So I noticed that the Album Art window (gen_classicart plugin) does not always show the artwork, even I have it available in the album's folder.
So after some experiments I deducted that it only shows either the art that's embedded in mp3 file, or it will show the image that's in the folder named "cover", "folder", "AlbumArt" or "<album's name>".
But if there is no embedded image or no image file named as mentioned above, it will show a blank window. Even if there's an album art image in the folder, just named something else.
It would be nice to have an option for it to show an image form the folder with any other filename if those default ones are not available.
It would still make sense to prioritize, firstly the embedded art, secondly those specific filenames but if those are not available fall back on showing any image file that's available.