It would be nice to be able to use .pngs in themes. Xpm doesn't handle transparency which it makes it hard to make claws look lovely. Also making it possible for themes to use pngs would probably attract more people making new and attractive themes for claws.
This is not yet possible as the summaryview and folderview needs xpm icons.
Implemented in 3.3.1cvs34 (Thanks Abhay the historian :)