How can I change the cover photo of an online album?

You can change the cover photo of an album online by:

Click on Photos > Find and click on an album. Then click Edit Details, and click on the Contents tab.

Once you do that, you will see a list of pictures that are in that album. The top picture will be your cover picture for the album. To change the order of the pictures, and to put a different picture as the first one, you can use the move up and move down options that are on the right.