As you work on your material, you must refresh the Storyboard thumbnails so that they will reflect the changes made to the image. You can refresh them anytime. You can refresh the current shot, or refresh multiple thumbnails at once.
To refresh a single Storyboard thumbnail:
The thumbnail border turns red if you are on Playhead A, or purple if you are on Playhead B.
To update multiple Storyboard thumbnails simultaneously: