Yes, you can choose the thumbnail that you want to represent an album.
Go to the Manage tab, open a collection so that the albums are listed. In the right-hand column you will see 2 symbols - a piece of paper and an X. Click on the piece of paper to edit the album. At the bottom of the edit screen you can select which thumbnail you would like to have represent that album. Click on the box and the thumbs for the images in that album will appear in a small popup. Click on your choice for the album thumbnail, then click Update.
You can also do this at the collection level, so you can specify a thumb to represent a collection as well.