Some years ago, when I solely worked in Windows, I happily used Irfanview as image viewer. Under Linux I didn't found a real replacement for it, yet. I already tried Gwenview, Kuickshow, Kview, ImageMagick's display, xv and maybe some others, but all have their little quirks and annoyances concerning speed or usability.
Here's a new candidate for my image viewer list: Feh (available for Ubuntu in the universe repository). Linux.com has a short overview of feh. It's a command line application, so it has it shortcomings on the usability front, but it quite fast and has nice features (hot keys, different view modes, mouse control, directory traversal, ...).