How to specify that cover art for a video in iTunes should be a frame from a specific time in the video?
The frame that's used is called the "Poster Frame". It looks like iTunes chooses a frame 10 seconds into the video by default.

To choose a specific frame as the cover art:


Start playing the video.
Pause the video on the frame you want to use.
Right-click on the video and choose "Set Poster Frame".