http://www.zenphoto.org/documentation/classes/Album.html#methodgetAlbumThumbImage
Note this is a album class method which means you have to create an album object of the album to get the thumb from first.
Well, you need to learn the php basics of working object orientated. There are also some typos:
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The parent gallery to be passed as the first parameter means the gallery object which is stored in $_zp_gallery on the theme pages. It is NOT the parent album! In Zenphoto terms the term "gallery" refers Zenphoto in total, not to an album (see the glossary, too). Note: If you trying to create an object from a subalbum you need to pass the full name with the parent folder like toplevelalbumname/subalbumname since you can have albums of the same name within different albums.
Also getAlbumThumbImage($album) does nothing. You have either to do $thumbimage = getAlbumThumbImage($album), then echo the variable or echo getAlbumThumbImage($album). Note that if no album thumb is set this function returns an image object.