nib - Interface Builder files
Nib files does usually not contain images. They are separately stored in the resource directory. They contain the layout of of the user interface of applications.
One very useful use of nib files is that they allow for translation of applications to other languages. You can do this with iLocalize.
To inspect many nib files you can download XCode from Apple, or find it on one of the CD's which came with your Mac.
You need to sign op to Apples free online developer program first: Apple Developer Connection