Whenever i uploaded pictures in an album i never had an option to change the album cover. Even if you try to edit the album or its settings it doesnt give you that option. The organize option of the google+ gives the option to organize the photos in the first page but not all the pictures of the album. The only way you can do this is by uploading via google picasa application installed in your computer. You have to first drag the pictures around and arrange the in an order that you want and keep the picture that you want as the album cover in the beginning of this order and then click on share on google+. You can initiate this by right clicking on a bunch of pictures that you have selected and then click on add to album>new album and publish it as an album in picasa and then click on share on google+.
