Hi! I think the best way to do it is basically to create two sections / blocks. The first will be a block video that will be hidden in phone view with a @media only screen and (min-width: ---px). The second will be a block image with a @media only screen and (max-width: ---px) that will be hidden in desktop views. All you have to do is to target the elements on the browser and include it on the media queries.
I hope this helps you.