If the scene in poser is dead straight usually I just use the photo as a backgound image. 

For stuff thats outside a window I shoot an image straight on - then place this on an plane or cube and align it level with the window but slighlty back. Then if the window scene looks "crooked" I just adjusting the plane to cope with that.

If I want using a background image like a sky and a photo then making an alpha plane with the alpha plane is the way to go.

One thing that can help a little is to dirty the windows a little with a transmap, as that can give sometimes give extra depth.   
 
One trick you can do if you want to use renders as background images is to create the background as if the wall wasn't there. Just leave a cube as a marker of where the wall will go.

Save the camera postion and your scene and render roughly using the same light set so the shadows match up. Render it. 

Now do a save as, delete all the background stuff and make your  scene up, move the camera around to line stuff up if needed, now replacing the marker with the wall and window. Import the 1st render as a background now load your 1st camera postion. Now render :) 

This way always work if you want a lot of figures in a scene and poser can't handle it. 

Pinky - you left the lens cap of your mind on again.