An album owner can choose an image as the album cover to help others with whom it's shared see what it contains.
First go to the Albums by following the link in the top banner. Then click on the album you want to choose a cover for.
Find the asset you want to use as the poster and select it by clicking on the checkmark next to its filename.
Now choose Use as poster in the action bar to set it as the album poster.
You can also set an album cover on the Albums screen. Simply hover over the album and click on the little image icon that appears in the top left. This opens up the selection window where you can choose a picture from any archive, album or your pinned items. You can even upload a new picture and use that as the album cover.