Answer Happy

HTML
<div id="container">
<img id="banner" src="http://via.placeholder.com/960x250"
alt="banner">
</div>
CSS
#container {
width: 960px;
margin: 0 auto;
}

@media (max-width: Xpx) {
#container {
width: Y%;
}
#banner {
width: Z%;
}
}