Imagemagick is currently my prefered X image viewer and command line image manipulation tool.
To set it as your default viewer goto /etc/mailcap.order and add/update the image line to display:images/*. After you've updated the file run update-mime. The command display handles most image formats
Don't save your primary images as gif this file format is on the way out, try png. png is supported by most web browsers (year=2000) but you might still want to use imagemagick's convert to turn your images into jpg or gif (if you need transparency) before you publish them.
Generally you dont want to use display to save images as the images have been scaled to fit the screen and this is the size they will be saved as - resulting in much larger files, for no gain