"Set as preview" (its a gig video edit option) saves a stretched image from the video even though the video's frame is correct

[audioboon]

I have uploaded a video with an image at the end that I want to use as preview image or thumbnail. The video fits the gig image frame perfectly and plays without a problem. But when I click “set as preview” on a given frame, the “preview” that is set is stretched and doesn’t fit the gig image frame.

Any advices how to fix this?