I think it is unavoidable, unless we show both the avatar of the topic creator as well as the last poster – which would be very busy – we have to pick either
- Topic creator
- Last poster
I think creator is more problematic because it will stay the same forever, even if there are hundreds of replies over a period of a week it would always be the same avatar displaying.
So I agree with @sam on this one it has to be last poster. Last poster is more dynamic and more interesting over time.