Short: use the save CLI command.
You can pull the image on a computer that have access to the internet.
sudo docker pull ubuntu
Then you can save this image to a file
sudo docker save -o ubuntu_image.docker ubuntu
Transfer the file on the offline computer (USB/CD/whatever) and load the image from the file:
sudo docker load -i ubuntu_image.docker
(On older versions this was just
docker load image.docker, see comments for more info.)