EBS backed AMIs are stored as EBS snapshots. EBS snapshots are stored on S3, but they "are not directly accessible through S3. They can only be accessed by creating an EBS volume." See the following thread: AWS Developer Forums: Snapshot Location
Backing up an EBS snapshot or AMI involves copying data from a volume using e.g. rsync:
- AWS Developer Forums: Backup Machine Images Off Amazon
- AWS Developer Forums: Can I download an EBS snapshot from S3?
An easier alternative would be to share the AMI with another AWS user (account). The other account would:
- launch an instance from the shared AMI
- create an AMI from the instance
Here is the documentation on sharing AMIs. It looks like a single command is required to share an AMI with another user: AWS Documentation » Amazon EC2 » User Guide » Using Amazon EC2 » Using AMIs » Sharing AMIs