
Setting the background image should be fine. If the browser / app allows the first frame as a poster image then it will override the bg image otherwise (i would think) the bg image would be displayed.
As per my suggestion above, setting the video to autoplay on iOS devices may give you a (1st frame) poster image. Therefore, not needing to create individual images. Maybe you could test that first. Failing that, return to the BG image approach.