Così sembra ok...

codice:
DIV#header {
	BACKGROUND-IMAGE: url(./img_center.gif); BACKGROUND-REPEAT: repeat-x; HEIGHT: 180px;
width:100%;
}
</STYLE>
</HEAD>
<BODY>
<DIV id=header>
[img]./img_sx.gif[/img]
[img]./img_dx.gif[/img]
</DIV></BODY></HTML>