More detailed information about -sDEVICE as follows:
Ghostscript provides a variety of devices for PNG output
varying by bit depth. For normal use we recommend png16mfor 24-bit RGB color,
or pnggray for grayscale. The png256, png16 and pngmono devices respectively
provide 8-bit color, 4-bit color and black-and-white for special needs.
The pngalpha device is 32-bit RGBA color with transparency
indicating pixel coverage. The background is transparent unless
it has been explicitly filled. PDF 1.4 transparent files do not
give a transparent background with this device. Text and graphics
anti-aliasing are enabled by default.
Options
The pngalpha device has one option.
The other png devices have no special options.
-dBackgroundColor=16#RRGGBB (RGB color, default white = 16#ffffff)
For the pngalpha device only,
set the suggested background color in the PNG bKGD chunk.
When a program reading a PNG file does not support alpha
transparency, the PNG library converts the image using
either a background color if supplied by the program
or the bKGD chunk.
One common web browser has this problem, so when using
<body bgcolor="CCCC00"> on a web page
you would need to use -dBackgroundColor=16#CCCC00
when creating alpha transparent PNG images for use on the
page.