First go to Smushit and enter the url of the image and it will squeeze a little more out of it.
Next make sure your css is called before any JavaScript or other files in your header.php
Check your CSS file and make sure the background image is being called at the top. You can also experiment with adding the image to html declration instead of the body by adding this line first in your css.
html {background:url(images/your-oversized-image.jpg) no-repeat center top;}
Also remember that the image is being cached by your browser so make sure to clear your cache to notice any changes in loading times.