You cannot add custom video thumbnails on our platform, but you CAN select from an auto generated gallery of thumbnails.
Visit your media center, which is the second button on the bottom navigation bar.
Hover over the video in question and then select “manage.”
Select “Other options.”
Select “Choose thumbnail.”
You can also add a graphic as the very first frame of your video, and then select from your account options to use that first frame by default. Just select "options" on the left hand menu bar and then expand "General settings."
