Use this css, as you already have the markup for it:
.img-container {
position: absolute;
top: 50%;
left: 50%;
}
.img-container > img {
margin-top:-50%;
margin-left:-50%;
}
Here is a working JsBin: http://jsbin.com/ihilUnI/1/edit
This solution only works for square images (because a percentage margin-top value depends on the width of the container, not the height). For random-size images, you can do the following:
.img-container {
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%); /* add browser-prefixes */
}
Working JsBin solution: http://jsbin.com/ihilUnI/2/edit