PNG images don't have -kb options
The PNG images for the documentation in the CVS in ogg/doc and vorbis/doc are
declared as text files in CVS, so when checking out on Windows platforms, where
the line ending is CR LF instead of a single LF the images are corrupt. To fix
this, set the -k'b' option on these files.
This also applies to the gif images in the mgm module.