Background Images Break Theme When Resizing Window

I am wondering what parameters I am not including when placing background images for my header, navigation, sidebar and footer when changing the Twenty Ten theme (or any theme for that matter).

I am merely trying to add my design to my web site, but if I resize the window (using Restore Down or anchoring to a corner or what have you) when the widest image reaches the edge of the window, the explodes the whole website, jamming the layout up and removing the content, sidebar etc.

Here is the code I’m using (for example) on my nav menu

#access .menu-header,
div.menu {
	font-size: 13px;
	margin-left: 12px;
	width: 928px;
	background: url(images/navigationshadow.png);

On an unrelated note, I am trying to remove the custom header image from Twenty Ten and I can’t seem to do it. I’ve tried the tutorial found here but that gives me a 505 error and breaks the site. At this point I’ve removed the header image in the Themes>Appearance but that leaves me with a big outline and the image not found logo, with my custom header image (in CSS) underneath.

Thanks