I looked at the pics and have a very important question, is the wall under the window plumb (Up and down) and if not, how far out is it? Use a 2 foot level if you can and tell us how far out of plumb in 2 feet it is by holding the level against the wall, then making the level read plumb, then measure how far the level is off the wall. OR is it bowed horizontally? If it is out of plumb it would appear to me that the walls around the window have settled and the area at the window has not. This will cause the bowing. It could be due to insufficient clearence at the header, in other words when it was framed the opening was too small and the window was forced into place.