No, they don't work in png(1) either:

term% hget http://9grid.es/screens/screen1.png|png
png: loadimage <stdin> of 2430000 bytes failed: loadimage: insufficient data

term% png n/dump/usr/akumar/plan9/screens/nihongonokawa.png
png: loadimage n/dump/usr/akumar/plan9/screens/nihongonokawa.png of 1310720 bytes failed: loadimage: insufficient data

(the latter is a snapshot of /dev/screen taken with topng(1))
ak