How do I browse submitted images? If it’s just for personal images why wouldn’t I just throw them up on S3?
Also you probably don't want every image you want to share on the internet coming from the same S3 bucket. And you don't want to pay S3/Cloudfront pricing so you'll probably want to put Cloudflare in front of it. Etc, etc.
But if you're happy with just using whatever S3 bucket you have, then there's no reason to switch.
For a Linux user, you can already build such a system yourself quite trivially by getting an FTP account, mounting it locally with curlftpfs, and then using SVN or CVS on the mounted filesystem.