[3] X Windows

XV is an excellent viewer for JPEG, GIF, and many other image formats.
It can also do format conversion and some simple image manipulations.
Get it from ftp.cis.upenn.edu:/pub/xv/xv-3.10a.tar.gz.  Shareware, $25.
Version 3.10 has some nifty new features, and it loads JPEGs noticeably
faster than any prior version.  If you're still using version 2.anything,
it's definitely time to upgrade.  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.

Another excellent choice is John Cristy's free ImageMagick package,
ftp.x.org:/contrib/applications/ImageMagick/ImageMagick-3.7.1.tar.gz.  This
package handles many image processing and conversion tasks.  The ImageMagick
viewer handles 24-bit displays correctly; for colormapped displays, it does
better (though slower) color quantization than XV or the basic IJG JPEG
software.

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.x.org:/R5contrib/xloadimage.4.1.tar.gz.  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.x.org:/contrib/applications/xli.1.16.tar.gz.