Basically, the same work (more-or-less) goes into modding Avernum 2's graphics as what I had to do with Avernum 3. The only difference in the file naming convention is that A2's files are prefixed by the capital letter 'G' for some reason.
Other than that little anomaly the naming conventions are the same. Portraits you modded for A3 can simply be ported lock, stock, and barrel into A2.
Link to the A3 thread: http://www.gamefaqs.com/boards/genmessage.php?board=918396&topic=50911234