Video Media Element – How to put in a background?

When I put my video in using the video element all you can change is the URL but I also need it to show a static image before it plays. I tried to use the shortcode with poster=”URL” but when I do that, it doesn’t perform right on mobile devices. It ends up not keeping the right aspect ratio and either goes off the page or plays on top and has back bars underneath/above. Do you know how I can fix this? I couldn’t find a “shortcodes for video” section anywhere on here which would be really helpful if you guys could stick that somewhere.