I have a bucket of images on S3. I set up the bucket contents to be delivered over a CDN using Cloudfront. Now if I add new images to the bucket do I need to re-setup Cloudfront for the bucket or are new items automatically distributed?
The items aren't automatically distributed, but there's nothing else you need to configure. The CDN nodes look locally for cached objects affiliated with that bucket. If the object doesn't exist but needs to be served from the CDN, the node requests it from S3 and caches it for a predetermined amount of time.
It's actually a lot like "DNS propagation", a common misnomer by which some people think DNS setting changes "propagate" around the world. In reality, the world's servers request the information then cache it locally. You're not waiting for DNS changes to propagate; you're waiting for the cached settings to expire. So it goes with CloudFront: New objects are served and cached when they're first requested. If you replace an object with a new object by the same name, CDN nodes that cached the object will show the old one until it expires.