yes it is possible. Lets say you put your image in wp-contents/uploads and it is called bg.jpg, ou will add the following to ATO->Add HTML/CSS Inserts->HTML Inserts: Body Top
<img src="./wp-content/uploads/bg.jpg" class="bg" />
and then add this to ATO->Add HTML/CSS Inserts->CSS Inserts
/* Set rules to fill background */
/* Set up proportionate scaling */
/* Set up positioning */
/* This is the only important rule */
/* We need our content to show up on top of the background */
neat little trick the guy came up with.
No guarantees it will work in all cases - this will cause the image to show behing the contents aslo, if you want the image to be hidden behind the container, add 'background: #ffffff;' to the container css.