This time, the script comes in a .deb package and it's called Cover Thumbnailer, and it displays music album covers, preview of the pictures which are in a folder, etc. in Nautilus. That means that it's no longer necessary to have a folder.jpg file inside a folder in order for the thumbnail to be displayed!
Cover thumbnailer is a small Python script which displays music album covers in nautilus, preview of pictures which are in a folder and more.
Download Cover Thumbnailer (Packages available for Ubuntu - .deb, and .tar.gz for other Linux distributions).
Note: after installing Cover Thumbnailer, you need to log out and log back in!