uhm, few nights ago I spent hours kicking a friend lj to have the header at the top of the generator layout...I succeeded in the end...I'm not sure I recall it right but in images, where you paste the url, I used "don't repeat" "top" and "scroll" instead in additional options I used 600 "center" and then I played with margins..I don't remember this banner height but be sure to use it as top margin ;)

dunno if that helped, sorry, I tried ;)