How to export EC2 snapshot into S3 bucket?
EBS Snapshots aren't really disk image files.  They're logical containers of pointers to chunks of compressed and deduplicated backup data that EBS stores in S3 when you create snapshots -- but these are stored in buckets that EBS owns and controls.

They are user-accessible via only one mechanism -- creating a new EBS volume from the snapshot.  When you do this, the new volume is linked back to its source snapshot and EBS starts loading the data onto the new volume in the background, from the snapshot storage in S3.  If you access a part of the volume that EBS hasn't yet restored, it immediately goes and fetches the chunks and puts them on the volume.

To create a disk image file of a snapshot, you create a new volume from the snapshot, attach it to a running instance without mounting it, and then use the same tools you would use for a standard hard drive -- tools like dd to read raw blocks from the snapshot, and write them to a file, which you can then download from the instance, e.g. using scp.