Each album can be provided with a thumbnail image (see example). If you want to keep it undistorted you have to use images with 80px width and 64px height.
(1) Click on the button "Browse" which is placed on the right of the field "Thumbnail image"
(2) It opens a standard window where you can find your image on your computer.
(3) Click on the name of the file and then on "Open"
(4) The path will be displayed in the "Thumbnail image" of your platform.