How do I change a video thumbnail in photos

I have been importing my digital scans of home videos into Photos. Some of my videos begin with a second or two of black until the image shows up, but the black image is shown as the thumbnail (or preview) when I see the video in the list in Photos.

Can this be changed to show a frame that I chose instead of the first frame of the video?

I'm thinking something similar to how albums have the option to "make key photo"