1. extract icons in OS X
press cmd-i on a folder or program. then click on the icon in the top of the window (blue selected) and press cmd-c. now open preview and press cmd-n to paste the icon. save the icon as a tiff file.
2. open edit and save with gimp
open the tiff file in gimp and edit the pic. then save the file as tiff – use „sichtbare ebenen vereinen / collaps layers“ – compression none.
sources for brand logos