The default image viewer in Ubuntu is a little slow for my liking but I quickly found a faster alternative in Viewnior. In Lucid just enter the following in the terminal to install it:
sudo apt-add-repository ppa:xsisqox/ppa sudo apt-get update sudo apt-get install viewnior
Once it is installed just right click on an image, select Open With from the menu and then select Viewnior as the default application.
Alternatively you can open Viewnior by clicking Applications, Graphics and Viewnior.
Viewnior is a great alternative to Irfanview which I have installed on my Windows box.