First, you're using the oldest version of this mod. While it works for its purposes, the newest version (2.0.1) has more power and flexibility. It does modify a different file (/includes/templates/your_template/tpl_main_page.php) and requires different naming conventions in the stylesheet.

In any case, the php code looks ok, and you say you are now getting the image. The css you show could not make the image appear anywhere but in the header. To stop the repeating, add

background_repeat: no-repeat;

to the declarations.