XV is one of the tools I use ALL of the time. It
is the most wonderful tool for displaying images of all types,
including GIF, TIFF, JPEG, BMP, X11 Bitmaps, Sun Rasters, IRIS
RGB images, and a whole pile of others. It can re-size images,
crop images, diddle the colors, do some simple image processing,
generate EPS files, capture X11 windows (great for incorporating
program screens in web pages or documentation). It is a
"must install" for all UNIX machines. At least as
important as programs like "more" or "man",
maybe more important.
XV is very easy to use -- you don't really need the manual. XV is
also expandable -- you can add custom features or add support for
odd file formats. There is version called XVscan that includes
support for scanners.