How will the result be used?   The only exception is a special case where image is ``<none>`` and tag is ``<none>``.  However then both are useless and we have to use the long/short ID anyway.  So, I think we will only ever see a single size per ``image:tag`` combo.  In other words, why require typing ``foo.bar[0] `` when just ``foo.bar`` is good enough?