Each album created in the Media Gallery has its own thumbnail image that displays on the page. By default, this is the first image listed within the album (i.e. the first item that displays in either Grid or Mosaic view).
To change the thumbnail to another specific image within the album:
Open the album and click on the "Edit Album" link on the top-right of the album window.
In the resulting pop-up window, click the "Use specified image" option under the "Album Thumbnail" section.
Next, click the "select" link to browse for and choose the desired image from the album. Once you have highlighted the desired image, click the "Select" button to proceed.
Finally, click "Update Album" to save your selection.