If this is the CSS body that you already have:

body {
     background-color: #0000CC;
} 

Then you just need to add it there:

body {
     background-color: #0000CC;
     background-image: url(3KQdNt.png);
     background-repeat: repeat-x;
     background-attachment: fixed;
} 

If you've already got a HTML file, you don't need the one that it has supplied you with - it's just an example. If you open it in a text editor, you'll see the CSS in the head section of the file. That's all you need, but you have to adapt it to your own page, like I've shown you above.

Note that this assumes that the .png image is in the same folder as the HTML file. If you place your image outside of that, then you will need the add the path to this bit of code:

     background-image: url(./your_folder/3KQdNt.png); 

I just resized the image and it worked. Thanks! Gradients are cool!