I just noticed that if you scale an image using width= and height= attributes of the img tag, Firefox 3.6 will (sometimes) put a thin border on the top and left of the image - something that you can’t get rid of with CSS/styling.
Weird - but a good rule seems to be scale your images to the correct size before using them in HTML.
This is different from the “border around link images” issue - which is easy enough to get around by using border-style: none in CSS.