I will try to phrase this question in a non-convoluted way. I make no promises.
Would it be difficult to vary the portrait by level, insofar as, if you wanted to use those pictures for level 90, but the old forum avatar pictures for race&class at levels 1-59, 60-69, 70-79, 80-89 respectfully?
http://www.wowwiki.com/Gallery_of_player_avatars
http://wowpedia.org/Gallery_of_player_avatars
ETA: Obviously someone's armoury picture (or something) would have to serve as a placeholder for certain excluded new(ish) race-class combinations. (e.g. Troll Warlock)
( This may be a dumb question, but my only forays into unit frame coding have been to tweak PerfectRaid when horribly out of date.
)