I can't recall if the Kindles have the 'file' command pre-installed in the system files or not ...
But if they do, a quick check:
Code:
core2quad Downloads $ file *.png
Orca.png: PNG image data, 600 x 800, 8-bit grayscale, non-interlaced
scr.png: PNG image data, 600 x 800, 8-bit colormap, non-interlaced