By default the first image in an Album will be selected as the cover image. You can easily change this to any image from that album. For the best Gallery Cover image results, a horizontal image is recommended.
When viewing an Album before sharing:
Select an Album, select the desired individual image that you would like to be a cover page. Choose "View Details" along the bottom and then select "Set as Album Cover" on the right panel.
When sharing an album as a Gallery:
Edit your cover page by selecting "Edit Gallery Features" on the left underneath the photo shown. Select "Change Cover Photo" to select a different image from the Gallery. Scroll through the images to find the desired one and select "Set as Album Cover" on the right panel. Next, Return the Edit Gallery features page and drag the target over the image below to position the cover photo.
Removing the Gallery cover image:
You can also remove the Gallery Cover image from your shared Galleries under the Gallery Configuration Settings.