As cegaton commented in your question the easiest way is to select all the faces, unwrap, and click on UV reset
but I'll try to explain it on how to do it and another useful thing when doing UV Mapping.
your uv map looks something like this:
The orange squares on the UV map represent the faces, as you can see here the faces don't cover the whole image and for that we will need to do UV Reset, just do U and click on "Reset"
Now that we reset the UV you can see that all the faces cover the entire image which is what we want, yay!
Now let's say that you only want a 3rd of the window, for that we need to do UV Map editing, just as objects in edit mode on the viewport we can select vertices, edges, and faces, for this we'll select the vertices we want to move and locate them on where we want
hope it helps