Thumbnail images are automatically assigned for all files. They can be manually assigned for folders and any item other than an image file. Items which are image files default to a thumbnail of themselves.
Navigate to the "Media Management" tab in Airship and select "Media Management"
Find the desired folder or item.
Click the edit icon.
In the thumbnail section, click the "Download File" button.
Alternatively, right click on Download File and select save link as...
Specify location to save the file. Click save.