iPhone CSS Sprite Issues

Hi I am designing a site with a CSS sprite in it. It looks fine in everything except iPhones, in which the background sprites are either zoomed out, or the positioning is completely wrong. Is this a know issue with iPhones? What’s the best way to solve it?

At worst I guess I could put in a media query and use separate images instead of sprites for iPhone but would prefer to know more about the issue and solve it properly.
Pasted below is the relevant CSS.


#site-wrapper {
	border: none;
	background:url(Images/Artwork.jpg) repeat-x center -420px;

#screen {
	background:url(Images/Artwork.jpg) no-repeat top left;
	width: 670px;
	height: 420px;
	margin: 40px 0 0 167px;
	float: left;

Just wondering, could it be connected with Retina displays?