ZIUUUUUM, v. 2

private double ModifiedImageZoomTransform = 1, ModifiedImageXTransform = 0.5, ModifiedImageYTransform = 0.5;
(...)
private void ZoomSliderValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
        {
            ModifiedImageZoomTransform = e.NewValue;
            ImageModified.RenderTransformOrigin = new Point(ModifiedImageXTransform, ModifiedImageYTransform);
            ImageModified.RenderTransform = new ScaleTransform(e.NewValue, e.NewValue);
        }
        private void PanXSliderValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
        {
            ModifiedImageXTransform = e.NewValue;
            ImageModified.RenderTransformOrigin = new Point(ModifiedImageXTransform, ModifiedImageYTransform);
            ImageModified.RenderTransform = new ScaleTransform(ModifiedImageZoomTransform, ModifiedImageZoomTransform);
        }
        private void PanYSliderValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
        {
            ModifiedImageYTransform = e.NewValue;
            ImageModified.RenderTransformOrigin = new Point(ModifiedImageXTransform, ModifiedImageYTransform);
            ImageModified.RenderTransform = new ScaleTransform(ModifiedImageZoomTransform, ModifiedImageZoomTransform);
        }