http://www.wizards.dupont.com/cristy/ImageMagick.html
and obtain the software from
ftp://ftp.wizards.dupont.com/pub/ImageMagick/
It is a little complicated to set up, as it requires several other
packages, but it will convert virtually any image format into another,
and permit a wide variety of operations on them.
For HPGL files, I have used the much simpler program hp2xx, which will
convert the HPGL to a wide variety of other formats. It is available
from most gnu sites.
I hope this helps.
-- Steve Philson philson@nmr.chem.umn.edu Director NMR Lab 612-626-0297 Chemistry Dept. University of Minnesota