The default image viewer in Ubuntu Gnome is eog (Eye of Gnome).If you feel eog to be cumbersome,and want a lightweight image viewer instead,you can try Viewnior.

Viewnior is a fast and simple image viewer for GNU/Linux. It only depends on gtk+-2.0.

Features:
* Fullscreen & Slideshow
* Rotate, flip, crop, save, delete images
* Animation support
* Browse only selected images
* Navigation window
* Set image as wallpaper (only under GNOME, see INSTALL file)
* Simple interface
* Configurable mouse actions

Install:

Viewnior can be installed from PPA,open a terminal window from Applications -> Accessories menu,and run following commands:

sudo add-apt-repository ppa:xsisqox/ppa
sudo apt-get update
sudo apt-get install viewnior

After that,right-click on and image file and select “Properties”,choose Viewnior at “Open with” tab.

The Homepage of Viewnior:http://xsisqox.github.com/Viewnior/index.html