The "Library" window in Interface Builder provides a "Media" tab that lists various images and sound resources that you can refer to from your nib file. There is a "System Media" item that lists default images provided by OS X, and if your nib is part of an Xcode project, you can also browse through images/sounds included in your Xcode project.
There is also a third item called "User Media", but for me at least, nothing is listed under there, and there doesn't appear to be any way to add new items. I've tried dragging images to the window, to no effect, and can't find any obvious location on the hard drive where such images might be stored. How would I go about adding items to this list so I can use them in Interface Builder?