A poster image is the still image used as the thumbnail and preview for the video when it isn't playing. By default this image is set to the first frame but it can be changed to a more suitable frame if desired.
Step-by-step:
Preview the image
1. Select the video file you want to view.
2. Click View in the right-hand sidebar or double-click on the file.
Set the Poster Frame
1. Click on the video to activate the controls.
2. Move the play-head to the frame you want to use as the poster image
3. Click on the cog icon to see settings menu and choose "Set poster image"
Due to the way that videos are encoded it is sometimes impossible to use the exact video frame chosen. The system will use a nearby usable frame instead.
