On windows (1.20b-beta, libpng-1.2.7), ????4a??.png were ng and ????6a??.png were ok:
here it seems that pdftex currently crashes on Willem van Schaik PNG test files
basn4a08.png 8 bit grayscale + 8 bit alpha-channel basn4a16.png 16 bit grayscale + 16 bit alpha-channel
ng
basn6a08.png 3x8 bits rgb color + 8 bit alpha-channel basn6a16.png 3x16 bits rgb color + 16 bit alpha-channel bgan6a08.png 3x8 bits rgb color, alpha, no background chunk bgan6a16.png 3x16 bits rgb color, alpha, no background chunk
ok
bgbn4a08.png 8 bit grayscale, alpha, black background chunk bggn4a16.png 16 bit grayscale, alpha, gray background chunk
ng
bgwn6a08.png 3x8 bits rgb color, alpha, white background chunk bgyn6a16.png 3x16 bits rgb color, alpha, yellow background chunk
ok -- Akira Kakuto