GridViewItem Dynamic Image

Question

Hey. i have a big problem.. i need to load an image into a grid view item from a external source for instance from the C:// drive or desktop how ever it does not seem to load.. it gives no errors:
          imgBrsh.ImageSource = new BitmapImage(new Uri("ms-appx:///Assets/Logo.png"));
        this line works and will load an image how ever using
        imgBrsh.ImageSource = new BitmapImage(new Uri("F:\Pictures\55797441.jpg"));
    however the above line does not load the image. i cannot use "ms-appx:///" since the         images will not be included in the project

any help will be greatly appreciated