I thought that JA might be referring to Elizabeth's viewing of the family portraits when she wrote that Elizabeth "knew nothing of the art." She did stop and contemplate it when she found Mr. Darcy's portrait (for all the reasons JA wrote), but it would not be particularly interesting to view family portraits, unless they were of your own ancestors, or the paintings were particularly distinguished in some other manner. If that is the case, then assuredly, Georgiana's drawings would be much more interesting for Elizabeth. Just a thought.