Cover Thumbnailer is a tool to get Nautilus, Caja and Thunar file managers to show folder thumbnails for music and image folders.
For music folders (needs to have cover.jpg/png inside the folder), the application lets you choose the thumbnail resize method (crop or preserve), and if to allow mosaic or not. For the pictures folder, you can choose the maximum number of pictures to show on the thumbnail. Besides the default Pictures and Music folders, you can add extra folders, and ignore folders if you wish.