$ gs -sDEVICE=pngalpha -o <output-%04d.png> -r1200 -dFirstPage=<start page idx> -dLastPage=<end page idx> <input>
n.b., the resolution of 1200 pixels for inch is quite large. Probably one wants to convert the result in smaller files.
$ gs -o noimage.pdf -sDEVICE=pdfwrite -dFILTERIMAGE <input>
$ convert <input> -colorspace HSL -channel L -separate <output>