I know there is a way to upload to S3 directly from the web browser using POST without the files going to your backend server. But is there a way to do it from URL instead o
I found this article with some details. You will probably have to modify your buckets' security settings in some fashion to allow this type of interaction.
http://aws.amazon.com/articles/1434
There will be some security issues on the client as well since you never want your keys publicly accessible