In order to use "BusyBox" image, one has to create an account in docker.io. service. https://www.docker.com/. Otherwise, pod creation will be blocked and the following message will be displayed:

Failed to pull image "busybox": rpc error: code = Unknown desc = Error reading manifest latest in docker.io/library/busybox: toomanyrequests: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit

Having docker.io credentials, the next step is to register them in OpenShift to be used while pulling images from docker.io. It is very easy using OpenShift console.
Go to: OpenShift console -> openshift-config project -> Secrets -> pull-secret -> Actions -> Edit Secret
Go to the bottom, *Add Credentials", fill the form, and it is done.

Another method is to pull docker.io image manually and push it to OpenShift internal registry.
https://docs.openshift.com/container-platform/4.5/registry/accessing-the-registry.html

Check accessibility of OpenShift internal registry.

nc -zv image-registry.openshift-image-registry.svc 5000

Ncat: Version 7.70 ( https://nmap.org/ncat )
Ncat: Connected to 172.30.108.134:5000.
Ncat: 0 bytes sent, 0 bytes received in 0.02 seconds.

If not available, log on to any of OpenShift nodes and do the same.

Log in to OpenShift as admin user

oc login https://api.openshift.cluster.com:6443 -u admin -p secret

Log in to docker.io

podman login docker.io

Pull the image to local podman registry

podman pull docker.io/rook/ceph:master

Retag

podman tag docker.io/rook/ceph:master image-registry.openshift-image-registry.svc:5000/openshift/rook/ceph:master

Login to OpenShift internal registry:

oc login -u kubeadmin -p <password>
podman login -u kubeadmin -p $(oc whoami -t) image-registry.openshift-image-registry.svc:5000

Test.

podman search image-registry.openshift-image-registry.svc:5000/

INDEX                              NAME                                                                                          DESCRIPTION  STARS   OFFICIAL  AUTOMATED
openshift-image-registry.svc:5000  image-registry.openshift-image-registry.svc:5000/openshift/apicast-gateway
openshift-image-registry.svc:5000  image-registry.openshift-image-registry.svc:5000/openshift/apicurito-ui 
openshift-image-registry.svc:5000  image-registry.openshift-image-registry.svc:5000/openshift/cli
................

Push to registry

podman push image-registry.openshift-image-registry.svc:5000/openshift/rook/ceph:master