Ah, I don't know. But I can tell you that ImageMagick is going to be a better tool for that. You'll want to look into the convert or mogrify utilities that are part of that package.
cbell@circe:~$ dpkg -s imagemagick
Package: imagemagick
Status: install ok installed
Priority: optional
Section: graphics
Installed-Size: 4464
Architecture: i386
Version: 7:6.3.7.9.dfsg2-1~lenny1
Replaces: geomview (<= 1.8.0), imagemagick-doc
Depends: libbz2-1.0, libc6 (>= 2.7-1), libfontconfig1 (>= 2.4.0), libfreetype6 (>= 2.3.5), libice6 (>= 1:1.0.0), libjpeg62, liblcms1 (>= 1.15-1), libmagick10, libsm6, libtiff4, libx11-6, libxext6, libxt6, zlib1g (>= 1:1.1.4)
Conflicts: imagemagick-doc
Description: image manipulation programs
Imagemagick is a set of programs to manipulate various image formats
(JPEG, TIFF, PhotoCD, PBM, XPM, etc...). All manipulations can
be achieved through shell commands as well as through an X11 graphical
interface (display).
.
Possible effects: colormap manipulation, channel operations, thumbnail
creation, image annotation, limited drawing, image distortion, etc...
cbell@circe:~$