If you want the background image to stretch to fit the entire element, you can set the background-size property to 100% 100%:
Try resizing the browser window, and you will see that the image will stretch, but always cover the entire element.
<style>
body {
background-image: url('cat.jpg');
background-repeat: no-repeat;
background-attachment: fixed;
background-size: 100% 100%;
}
</style>
Live Demo & Try it yourself!