PNG (Portable Network Grapics) file is bitmap image format, wich supports losless compression, palette and full-color images with or without alpha chanel transparency.
The file format does not support layers.
Here is the file type:
$ file sample.png sample.png: PNG image data, 48 x 48, 8-bit/color RGBA, non-interlaced
Sample full-color png file: download here.
$ file sample_256.png sample_256.png: PNG image data, 48 x 48, 8-bit colormap, non-interlaced
Sample 8-bit colormap png file: download here.
$ file sample_greyscale.png sample_greyscale.png: PNG image data, 48 x 48, 8-bit gray+alpha, non-interlaced
Sample greyscale png file: download here.