Go to Gallery >> Manage Gallery >> [select the gallery you'd like to edit thumbnails], mid-way down the screen you’ll see a list of all your images. Hover over a thumbnail and you’ll see a row of links appear. Click on the link that says ‘Edit Thumb’. A window will pop up with an option to select the area of your image you’d like to be the thumbnail. Click save changes.
*If you do not see this link, open up your site’s Dashboard in the Mozilla Browser
