An image lives in a registry, a server that speaks the OCI Distribution API (OCI Specifications). Name an image without a registry and Docker 514 assumes Docker Hub 514 (docker.io); leave out the namespace and it assumes library, home of the Docker Official Images:
Official Images are built in the open from docker-library/official-images (https://github.com/docker-library/official-images 7,008 ). Docker Hub also badges Docker Hardened Images 514 , Verified Publishers (vetted vendors) and Sponsored OSS projects; any other namespace is unreviewed, and docker search postgres marks the official result [OK]. Hub limits pulls to 100 per 6 hours per IP address anonymously and 200 with a free Personal account (paid plans are unlimited), which is why Image Registries logs in.