its exactly explained on this site!
put the css in your template css file.....
the table with the backgroundimage (header) can you maby put in a div ("header" for eg)
change the path from the img in the css with the WB variable
for eg.
background:url(./img/bg_head_top.jpg);
try the funktion eventually on a code2 page, then you can see that this technic is working....and then implement it to the template!
regards martin
also read the help documentations befor editing your template if you are not so skilled with this things!