XV is an excellent viewer for JPEG, GIF, and many other image formats.
It can also do format conversion and some simple image manipulations.
Current release is 3.10a, available from http://www.trilon.com/xv/ or from
ftp://ftp.cis.upenn.edu/pub/xv/.  Shareware, $25.  HINT: if you have an 8-bit
display then you need to "lock 8-bit mode" to get decent display of JPEG
images.  (But do NOT do this if you intend to resave the image, because
it'll be written from the 8-bit version, thus costing you image quality.)
You can set this mode to be default by adding "xv.force8: true" to your
.Xdefaults file.  To override that default for editing, say "xv -24".

Another excellent choice is John Cristy's free ImageMagick package, currently
at release 4.1; see http://www.wizards.dupont.com/cristy/ImageMagick.html.
This software handles many image processing and conversion tasks.  The
ImageMagick package provides a C/C++-callable library and a set of command
line processing/display programs.  Perl and Python interfaces to the
ImageMagick library are also available.

Both of the above are large, complex packages.  If you just want a simple
image viewer, try xloadimage or xli.  xloadimage views and converts many
image file types including JPEG.  Version 4.1 has better JPEG support than
prior versions and is easier to install.  xloadimage is free and available
from ftp://ftp.x.org/R5contrib/.  xli is a variant version
of xloadimage; xli is slightly better as an interactive viewer, but it can't
be used as a converter, and it supports fewer file formats.  xli is also
free and available from ftp://ftp.x.org/contrib/applications/.