You say this:
ppmtogif file.ppm -transparent rgb:f/f/f > file.gif
to produce file.gif with white marked as a transparent color. Then
modern GIF viewers, including X Mosaic, will show the background color
instead of white. See the manual page for more details.
I don't know the canonical place to get NetPBM; try archie.
--Mark Torrance
Mark.Torrance@east.sun.com