Image size distors on mobile

Sorry for the late response. The free version does not knows how to serve alternative banners. What you can do is wrap your banner with a div with a css rule that will limit it’s height in a mobile device or based on screen size
For example <div class=”mobile-rules”>Banner</div> where mobile-rules is taken from somewhere like this https://www.htmlgoodies.com/beyond/css/targeting-specific-devices-in-your-style-sheets.html