better off try to avoiding "position" attribute and negative margins at all in css (different browsers most commonly produce different results, when applied)...
use this instead:
CSS
img#topright {
margin:0 0 auto auto;
border:0;
float:right;
}
html
<img id="topright" src="..." width="..." height="..." alt="..." />
EDIT: make sure your body margins and paddings are set to 0...
