Upload a custom video library file thumbnail via Upload from Computer:
Click the file options ellipsis.
Select Edit Thumbnail.
Click Upload from Computer.
Select a file and click Open.
Select a video library file thumbnail via Select From Stills:
Click the file options ellipsis.
Select Edit Thumbnail.
Click Select From Stills.
Select a thumbnail and click Save.
Generate a video library file thumbnail via Select from Video:
Click the file options ellipsis.
Select Edit Thumbnail.
Click Select from Video.
Navigate to the desired video frame and click Save.