To set a thumbnail for one of your image galleries, go to the Gallery Content Admin, locate the content set you wish to change, and click the "Gallery" or "Thumbnail List" action icon, which will be the second available action icon.
On the next page, you will be taken to a display of all available thumbnail images for that photo set in CARMA. You can set the default thumbnail for that set by clicking the "Set as Sample" action icon below your desired image. You can also only allow that image to be a thumbnail by clicking the "Restrict to thumbnail size" action icon.
Once you have set your default image thumbnail, it will be surrounded by a black border to indicate that it is the default thumbnail for that content set. You can change your image thumbnail as many times as you wish.