How to stop OS X from shuffling windows everytime I use Spaces?

How are you switching Spaces? In my case I use a hot corner to switch. When Spaces then displays itself, whichever window you click on in your destination Space is one that will appear on the top. So in your case, if you are using hot corners, when you want to return to the Space with Preview, make sure you actually click on the Preview window.

Does that help?

 

Try Hyperspaces?

I use Hyperspaces (which apparently sits on top of Spaces, or at least works
very closely with Spaces). I have mapped the F16 and F19 keys to allow
movement forward and backward through my Spaces. Whenever I land on a
Space, everything is in exactly the same state as when I was last in that
Space.

My $0.02 . . . ;^)