When you upload a video to Backlot, during transcoding, Backlot generates a series of thumbnails (also known as a "preview image") of the video at 15-second intervals.
Thumbnails are sized at 1/3 of the encoding width. By default, a thumbnail is taken at 15-second intervals throughout the length of the video. If you want to change this interval, contact Brightcove Technical Support. However,
the "preview image" is a global setting per video. Only one of the generated thumbnails (or one of the custom uploaded preview images) can be set as the one that is displayed in the player preview. By default, this is the first thumbnail from thehighest-resolution encoding.