Selectable thumbnails is in Beta 2 I thought. Just click the "edit" icon next to a collection or an album and you can choose which image you want to represent that collection or album.
Took me a while to find the selectable thumbnails option so I came searching *headdesk*. In a future release, might it be possible to get some kind of preview of the image you're selecting rather than just the filename?
ETA: Another nice extra would be the option to set a file as the default thumbnail during the upload process, even better if it was available during importing as well.