I deployed an image to Kubernetes, but it never becomes ready, even after hours.

$ kubectl get pods

NAME                            READY     STATUS             RESTARTS   AGE
myapp-b8dd974db-9jbsl    0/1       ImagePullBackOff   0          21m

All this happens with the Quickstart Hello app, as well as my own Docker image.

Attempts to attach fail.

$ kubectl attach -it  myapp-b8dd974db-9jbs

Unable to use a TTY - container myapp did not allocate one
If you don't see a command prompt, try pressing enter.
error: unable to upgrade connection: container 
   myapp not found in pod myapp-b8dd974db-9jbsl_default

Attempts to access it over HTTP fail.

In Stackdriver Logging I see messages like

skipping: failed to "StartContainer" for "myapp"
with ImagePullBackOff: "Back-off pulling image 
\"gcr.io/myproject/myapp-image:1.0\""

and No such image

Yet I did deploy these images and the Cloud Console shows that the pods are "green."

And kubectl seems to tell me that the cluster is OK.

$ kubectl get service myapp
NAME           TYPE           CLUSTER-IP     EXTERNAL-IP      PORT(S)          AGE
myapp   LoadBalancer   10.43.248.78   35.193.107.141   8222:31840/TCP   29m

How can I diagnose this?