png writes a png format image file of the current image to disk.
png filename, dpi
cmd.png( string file, dpi )
To obtain transparent png images, use the following command before ray-tracing and exporting your scene:
set ray_opaque_background, 0
Use the DPI option to have PyMol set the DPI of your image. If I execute
png /tmp/ex.png, dpi=300
I will get a four-inch square image at 300dpi. Leaving off the dpi flag would net me a 1200x1200 image at 72dpi. This saves the intermediate step of having to use GIMP/PhotoShop/etc to rescale your photos for publication.