The style img {width: 100%;} is being applied to images that you don’t want it applied to. Give the header image a class and then just apply the width 100% to that.
<style>
.stbc-header-image {
width: 100%;
height: auto;
}
</style>
<img class="stbc-header-image" src="https://www.dropbox.com/s/wtwkcf2jidfd92r/single-track-bc-
header.jpg?raw=1" style=padding-bottom:18px; alt="Single Track BC header
image">