JPEG is a widespread, semi-standard image format. But most of all, it is good enough, and it scales. There’s just no reason to switch to JPEG 2000 or some other “improved JPEG format” right now, except for very special applications you won’t find on the web.
Most people wouldn’t even notice the difference, unless they compare the pictures side-by-side like above. In cases where it WOULD make a difference, the JPEG creators just notch up the JPEG quality setting. Who cares wether it’ll take a couple kbytes more nowadays?
JPEG gets the picture across. In the example above, I see four times the same woman with a hat and purple thingamabob. I notice the difference in quality, but frankly i don’t care, because whatever format, i am still able to get the “message” of the picture regardless of the subjective minus 10% loss of image quality.
So, my point being: unless pr0n sites start using JPEG 2000, it’ll never become a standard format of the web.