We're all impatiently awaiting the Android 4.3 update for the Galaxy S3 (and the Note 2) that's supposed to hit in October, and to whet your appetite, an image of a Galaxy S3 running on the latest version of Android has been leaked by the folks over at AndroidSaS. The image has been blurred for the most part, but not enough to hide the fact that the device is on Android 4.3 (though it could also be a result of a bit of build.prop editing).
According to AndroidSaS‘ source, the Android 4.3 update will mainly be a security and bug fix update, and that it doesn't have any new features from the Galaxy S4. There's still time before the final update hits though, so we'll just have to wait it out for more details (and a leaked firmware) to see what the update really brings to Samsung's yesteryear flagship.
Update: On closer inspection, the image does seem to be fake, so don't take it too seriously. Android 4.3 is surely coming though.