Background size does not work on an image tag. It works for example on a div. What you need is to add the image as a background-image of a div and then apply background-size: cover.
See this fiddle: http://jsfiddle.net/BMvt8/2/
I have created a div with the class image and used it instead of the img tag:
<div class="image" style="background-image: url(http://i.imgur.com/iZcbLKf.jpg); background-size: cover"/>
And added to the css:
.image {
height: 320px;
width: 600px;
position: absolute;
top: 0;
left: 0;
z-index: 2;
}
.image_title {
z-index: 3;
}
The z-index is needed to put the image title on top of the image of the dog.