If you want a command-line JPEG conversion program, see the IJG source code
described in section 15.  (This code is included as a subdirectory in most
of the X programs described above, although they may not have the latest
version.)

Non-X viewers are hard to come by, since they are very hardware dependent.
Linux users with VGA/SVGA displays may like zgv.  Version 3.1 is available
from sunsite.unc.edu:/pub/Linux/apps/graphics/viewers/svga/zgv-3.1-bin.tar.gz.
(Several other alternatives are available in the same directory.)
If you use a less popular platform, you're probably out of luck.