OMEROFormat: store only image ID, not pixels ID

While it would be far more intuitive to store the pixels ID as well,
attempting to do so naively results in:

omero.UnloadedEntityException: Object unloaded:omero.model.ImageI@a3f556f

Rather than doing another server round-trip just to grab the pixels ID
too, let's store only the image ID. It will be up to downstream
consumers to query OMERO again for the pixels ID if needed later.