I'm having the same issue.
I added the below code to Page Settings > Advanced > Header.
It resized the banner fine, but now there's a big black square under the banner on mobile.
Something to do with the height. Help? http://www.xfallenmoon.com/
Oh, and it's not resizing on tablets either... not sure what I should add in the code for that.
<style>
@media screen and (max-width:640px) {
.banner-thumbnail-wrapper img {
width: 100% !important;
height: auto !important;
left: 0 !important;
}
}
</style>