Instead of using the images to round the corners (hard way). It would be better, quicker, easier, and more aesthetically appealing (smoother edges) to implement something like this:
http://www.dillerdesign.com/experiment/DD_roundies/
The probablem with using images to round the corners is at first the page displays the square corners, then renders the images, so for a second (sometimes longer) the user will see square sharp corners until the images render. This doesn't provide a good end-user experience as if the corners were rounded the whole time. It also eliminates the jagged corners presented using anti-aliased and resized corner images which is not aesthetically appealing.
Can something like this be added this to the nopCommerce main codebase? It should be fast.