Why go to the trouble of EC2 when Amazon S3 will do this for you, and is a LOT easier manage? Just use something like BucketExplorer for upload/download, and you don't need to worry about starting images, making sure they're still running, keeping software versions unto date, paying for idle time, etc.
Also look at Amazon's CloudFront service, which is a CDN for Amazon S3. It may or not be economic for you, but it can greatly improve download times for your users.