Looking at the grain in your excellent pictures, it doesn't appear to me that the corners of your quilt aren't square.
If they are square, then the problem is with the attachment of the binding.
Are you stopping the stitching that attaches your binding to the front at exactly the same distance from the edge as the seam allowance width and backstitching there? When you start the seam on the next edge after the corner, your line of stitching will touch (or nearly touch) the stopping point of the other seam. If those seams overlap or cross in the corner, your fabric won't turn correctly.
Another possibility is that when you make the fold in the binding at the corner, you may not be making a 45 degree angle with the fabric that folds under the stitching. A little wonky there can make a mess. Looking at your picture, I think that fold technique might be your problem.
I am hoping others can zero in on it and help you more.
Dayle
Last edited by Daylesewblessed; 10-27-2012 at 05:10 PM.