You'll want to lock the mouse on to the image to stop the cursor slipping off it Mouse.Capture (imageReference), you can release it on mouse up with Mouse.Capture (null) – Andy Feb 18 at 15:58
After using Andy's suggestion, by adding:
Mouse.Capture(img);
at the bottom of the MouseLeftButtonDown-function, and
Mouse.Capture(null);
at the bottom of the PreviewMouseLeftButtonUp-function, it works like a charm. So thanks a lot Andy!