Gwenview is another good standard kde image viewer. I hear it's gotten some nice new features in kde4 too.
There are several other good viewers out there that are agnostic of desktop dnvironment, like imgSeek and xnview. If you want something really fast and simple (but powerful), then you can even use imagemagick (available as 'display' and 'convert' commands, useful from a console). Wikipedia has a good comparison page
of available programs.