or any other program while i'm at it, ubuntu built-in utilities are nice but there are better.
There are two ways to do this.
The Easy Way
The first is to right click on a picture in your file manager, select Properties and then navigate to the Open With tab, then select the application you want and click Close:
The Second Way
You can also right click on files you want to change by right clicking on them selecting Open With -> Other Application ...
From there you will have the option on what program you want to use to open that file, select your preferred viewer from the list:
You need to make sure you check the bottom checkbox.
You can use Ubuntu Tweak to do this.
Once you have installed it, Applications -> System Tools -> Ubuntu Tweak -> System -> File Type Manager -> Image.
You can go to system->preferences->preferred applications in your menu to change many of your applications, but you can also right click your files (like image files), click properties, and then open with. You can choose a default application from here for your file type :).
There may be a gconf entry for the image viewer, but I'm not sure where it is, so maybe I will add that to my answer later.
Next question: how to change the default app for a batch of filetypes?
This annoying default filetype associations problem annoyed me so much that I wrote an article about it! It is a somewhat clumsy solution but it works.
Hope it helps!