The thumbnail image visible in the Folderpanel is mostly used as image in a Facebook and other share actions.
Via the "thumbPath" variable it is possible to get a link to the Thumbnail image shown on the Folderpanel (see here).
However that image is mostly to small to be displayed in Facebook and it is for Facebook not convenient that the image is not in the same directory as the index file of the album.
So please make in the jAlbum core software a folderthumb.jpg image in output directory the same way as the folderimage.jpg is made, so with dimensions supplied in the skin. In this case just giving the required width is sufficient I think.