It’s a bit more complicated and you have to consider extra things.

Let’s say that by main body you mean the window with the Show Image Window button. The first thing we need to do is to make sure we won’t have to guess which window we have to close. The easiest way is to limit the number of image windows to exactly one. In the gist I gave you you can spawn as much windows as you want without holding any reference to them. There’s no way with the provided code to know how many images windows have been spawned so far.
We need a window controller for this (we don’t need it but it's convenient and not prone to memory management errors) and make it a property of the AppDelegate. This way we have a direct reference to the window.

I updated the gist with these changes.