You can run an image depends on whether you gave the image a tag/name or not.
$ docker images
root@dockertest:~# docker images
REPOSITORY TAG ID CREATED SIZE
ubuntu 12.04 8dbd9e392a96 4 months ago 131.5 MB (virtual 131.5 MB)
With a name for e.g. - in this case Ubuntu :
$ docker run -i -t ubuntu:12.04 /bin/bash
or you can run it by just using the ID:
$ docker run -i -t 8dbd9e392a96 /bin/bash
I hope it would be helpful for you.