How to change the image that is displayed while the video is loading?

Video Background shows an image while the video is buffering the content to avoid showing the default loading animation.

The image that is displayed while the video background is loading is the YouTube or Vimeo video thumbnail.

It can be easily changed from your YouTube video manager:

Or from your Vimeo manager:

Usually selecting the first frame gives the best results because that way there's no change from the image to the video. 

See the YouTube documentation and Change your Create video's thumbnail - Vimeo for instructions on how to set a custom thumbnail.