"We found the window breaking..." is grammatically possible, but it's not a very useful sentence. It means that at the same moment we found the window, it was in the act of breaking. Windows break very quickly, so it's extremely unlikely you will ever find a window just as it is in the act of breaking.
But you can certainly imagine a window breaking. You can imagine watching a window as the glass shatters. You can even see a window breaking; you can be looking at it, and while you are looking at it... smash!.
If you imagine the window broken, that means you see, in your mind's eye, the window as it might look if it were already broken.