This assumes you have installed Docker on Linux and Docker is running.

A Docker image contains the code used to create a Docker container, such as creating a Nginx web server, or a mySQL server, or a home grown app, and the list goes on. In this way, an image is like a template used to create a container. An image is kind of like a virtual machine, but much more light weight, using significantly less storage a memory (containers are usually megabytes in size).

The docker images command will return the images you've downloaded.

docker images

Which will return something like this.

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
foo                 latest              105b54dc64f1        32 hours ago        196.7 MB
bar                 latest              95ga44bc5582        2 weeks ago         967 B

The docker info command can be used to return the number of images.

docker info
. . .
Images: 2