One might have hundreds of images that lifecycle policy missed (or none was set) and therefore getting reports on images that are not used with services is not as important as ensuring scanning the images currently used in task definitions.
It would be useful to be able to have a separate function that scans for task definitions, identifies the ones that have images in AWS ECR, and passes the images to scan and report on.
Allowing multi account by providing IAM Roles that can be used to go scan task definitions in other accounts