Amazon EC2 now supports tagging when registering or copying AMIs

Amazon EC2 now enables you to tag your Amazon Machine Images (AMIs) when you create an AMI from an EBS snapshot, or when you copy an AMI within the same or different AWS Regions. Tags are simple key-value pairs that you can assign to AWS resources such as AMIs to easily organize, search, and identify resources, create cost allocation reports, and control access. 

Source:: Amazon AWS