The benefit of this is that the image replacement stuff happens AFTER the page has been loaded for snappier response times. The page looks better with image headlines because you're not font-limited there (see apple.com for example). And most importantly: you want images for headlines but you also want to be found on Google.
Go to the demo page to see it in action.
This is of course only a proof of concept demo. If you do this properly you'll want to do two things properly unlike what I've done:
The script that generates this image is very fast. On this machine it takes about 0.02 seconds and with run-once-only caching in place that's no time at all.