CSS3 specifies a new image() function that can be used to specify fallback images to be used when a more-preferred image can't be loaded/displayed. It would be good for WebKit to support this.
See http://dev.w3.org/csswg/css3-images/#image-notation for more details.
Forward-duping to a bug that has some work done on it.
*** This bug has been marked as a duplicate of bug 72584 ***