How can the image name in this docker compose file be found?

I wrote a docker-compose.yml file referencing this tutorial: Running PostGraphile in Docker, but when I run this command docker-compose pull && docker-compose up -d to update an image an error occurred:

WARNING: Some service image(s) must be built from source by running:
    docker compose build graphql
Error response from daemon: pull access denied for forum-example-graphql, repository does not exist or may require 'docker login': denied: requested access to the resource is denied

But if I run docker-compose up -d, everything is OK.

I wonder how the forum-example-graphql image in the following snippet can be found(from docker hub?)?

graphql:
    container_name: forum-example-graphql
    restart: always
    image: forum-example-graphql
    build:
        context: ./graphql
    env_file:
        - ./.env
    depends_on:
        - db
    networks:
        - network
    ports:
        - 5433:5433
    command: ["--connection", "${DATABASE_URL}", "--port", "5433", "--schema", "public", "--append-plugins", "postgraphile-plugin-connection-filter"]

I learned from here that the image name in the compose file is based on the created image name, but I did not name the local image as forum-example-graphql.

In another service the configuration is as follows:

  postgres:
    container_name: postgres
    image: postgres:latest
    restart: always
    volumes:
      - "./postgres-data:/var/lib/postgresql/data"
    networks:
        - network
    ports:
      - "5432:5432"

The image value is set as postgres:latest and the image is pulled directly from docker hub using this config, given no context and Dockerfile.

Then I wonder how the local image is named and how does it relate to the service image value.