[Last modified: Oct 94]

Answer: An application called "xv" (interactive image display for the X Window
System) is useful for displaying and converting many image formats. From the
man page:

xv is an X11 program that displays images in the GIF,  JPEG,
TIFF,  PBM, PGM, PPM, X11 bitmap, PDS/VICAR, Sun Rasterfile,
and PM formats on 1-, 2-, 4-, 6-, 8-, 16-, 24-, and 32-bit X
displays.   xv  will also read compress-ed versions of these
files.

You can get "xv" (shareware by John Bradley et al) from:

ftp://ftp.cis.upenn.edu/pub/xv
or:
ftp://ftp.x.org/R5contrib/

Another useful conversion package is "pbm" (portable bitmap file format) by
Jef Poskanzer et al, available from:

ftp://ftp.x.org/R5contrib/
or:
ftp://ftp.x.org/R5contrib/ (much older :-)

You might also want to check the X11 FAQ for additional conversion options:

ftp://ftp.x.org/contrib/faqs/FAQ