This is easily done and the image extends down into the content area which is what I wanted.
In 'Body text & Links' i pasted
Code:
background: url(/wp-content/themes/atahualpa/images/bg_main.jpg) repeat-x;
In 'Add HTMML/CSS Inserts' I pasted
Code:
#container{ background: url(/wp-content/themes/atahualpa/images/bg_laptop.jpg) no-repeat; }
No URL because it's only on a test site which will be wiped.
I searched here and couldn't find the answer so have posted this, hope it helps someone.