The rollover image is already loaded as the background image of the p tag that contains the a tag with the no-roll image (view source). It does not need to be loaded (or, in the case of Internet Explorer, reloaded) on hover, so there is no flicker or flash of white before the rollover image appears.
On rollover (CSS hover), the link background CSS is set to none, revealing the background image of the p tag beneath. Yep, it's a pig-ugly image, but good enough for a demo.
Here's another example using a typical 3rd party link (in this case, Reddit):