Send an email to artists (at) istockphoto, attach the better version of the image and put "Replacement request for file #xxx" in the subject...
New image will have to be inspected manually, that's why it shouldn't be overused.
Best regards,
Michael
Sidenote: With site specific requests, I'd be sure you would get a faster correct answer in the site forums. Keep cross-site discussions and "blame the best match" threads here, of course. ;-)