How can I change the default program for viewing pictures and photos?

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.

Go to the:

System Settings > Details > Default Applications

And set Photos