A thumbnail is a single frame in a video that is used as the preview, or still image when a video is loaded.  The system selects a frame from the video for your default thumbnail selection when uploaded.  Each video has one thumbnail.

To change the thumbnail for your video, the video must first be uploaded. 

From the Videos tab, select the Edit button on the video for which you want to change the thumbnail.

Play the video then pause at the frame you wish to use for your thumbnail. Click the Thumb button on the player bar.

When thumbnail selection is saved you will see the message, "New thumbnail image saved."