Make image responsive. Make it fit to div - html

Question

I found this code on https://thecodeplayer.com/index.php/walkthrough/3d-thumbnail-image-hover-effect and I want to make the image both responsive and whenever I change an image, the image has to scale to the div.
I tried to set: width: 100%; and height: auto; but it did not work. Is it because the HTML does not contain <img> tags? As I'm trying to learn I would appreciate any advice you have for me on how to solve this.
/*custom font - Montserrat*/
#import url(http://fonts.googleapis.com/css?family=Montserrat);
/*basic reset*/
* {margin: 0; padding: 0;}
/*forcing the body to take 100% height*/
html, body {min-height: 100%;}
/*a nice BG*/
body {
background: #544; /*fallback*/
background: linear-gradient(#544, #565);
}
/*Thumbnail Background*/
.thumb {
width: 400px; height: 300px; margin: 70px auto;
perspective: 1000px;
}
.thumb a {
display: block; width: 100%; height: 100%;
/*double layered BG for lighting effect*/
background:
linear-gradient(rgba(0, 0, 0, 0.4), rgba(0, 0, 0, 0.4)),
url("http://thecodeplayer.com/u/m/i1.png");
/*disabling the translucent black bg on the main image*/
background-size: 0, cover;
/*3d space for children*/
transform-style: preserve-3d;
transition: all 0.5s;
}
.thumb:hover a {transform: rotateX(80deg); transform-origin: bottom;}
/*bottom surface */
.thumb a:after {
/*36px high element positioned at the bottom of the image*/
content: ''; position: absolute; left: 0; bottom: 0;
width: 100%; height: 36px;
/*inherit the main BG*/
background: inherit; background-size: cover, cover;
/*draw the BG bottom up*/
background-position: bottom;
/*rotate the surface 90deg on the bottom axis*/
transform: rotateX(90deg); transform-origin: bottom;
}
/*label style*/
.thumb a span {
color: white; text-transform: uppercase;
position: absolute; top: 100%; left: 0; width: 100%;
font: bold 12px/36px Montserrat; text-align: center;
/*the rotation is a bit less than the bottom surface to avoid flickering*/
transform: rotateX(-89.99deg); transform-origin: top;
z-index: 1;
}
/*shadow*/
.thumb a:before {
content: ''; position: absolute; top: 0; left: 0;
width: 100%; height: 100%;
background: rgba(0, 0, 0, 0.5);
box-shadow: 0 0 100px 50px rgba(0, 0, 0, 0.5);
transition: all 0.5s;
/*by default the shadow will be almost flat, very transparent, scaled down with a large blur*/
opacity: 0.15;
transform: rotateX(95deg) translateZ(-80px) scale(0.75);
transform-origin: bottom;
}
.thumb:hover a:before {
opacity: 1;
/*blurred effect using box shadow as filter: blur is not supported in all browsers*/
box-shadow: 0 0 25px 25px rgba(0, 0, 0, 0.5);
/*pushing the shadow down and scaling it down to size*/
transform: rotateX(0) translateZ(-60px) scale(0.85);
}
<!-- Container -->
<div class="thumb">
<!-- Image -->
<a href="#">
<!-- Label -->
<span>Three-eyed Robot</span>
</a>
</div>