Are you sure its working?
In my tests (in Chrome) the image is not loaded the first time you try and print the page. If you try a second time then the image appears.
The image seems to need to be in cache before the print is selected. The only way I can see that that will happen is by loading it by default in the main stylesheet and hiding it offscreen and then just revealing it in the print stylesheet. This of course means the image is loaded when it may never be needed.
Are you getting different results?