Inspect the root folder of your music storage for picture files. Probably .tbn, .bmp, or .jpg files.
They might be marked as “System” or “hidden”, so enable the appropriate options in whatever you use for file management to see them.
Remove them, so they are not used by Plex.
Here is official documentation, where is expecting to find any pictures to use for artists or albums: https://support.plex.tv/articles/215916117-adding-local-lyrics/
Pictures outside of these locations – particularly higher up in the folder hierarchy – should be removed.
