private String imageReferenceToImageId(ImageReference imageReference){
  return imageReference.getPublisher() + ":" + imageReference.getOffer()+ ":"+ imageReference.getSku()+ ":"+ imageReference.getVersion();
}

private String imageReferenceToImageId ( ImageReference imageReference ) { return imageReference . getPublisher ( ) + ":" + imageReference . getOffer ( ) + ":" + imageReference . getSku ( ) + ":" + imageReference . getVersion ( ) ; }
converts image reference to image identifier .
