Printing a WPF BitmapImage

Building upon Drew's answer, it is best to measure and arrange the container that is handed to the PrintVisual method. This will prevent an image that is larger than a 8.5 x 11 sheet of paper from being cut off. Here is an example of how I printed an image that was partially visible on-screen:

PrintDialog dlg = new PrintDialog();
bool? result = dlg.ShowDialog();

if (result.HasValue && result.Value)
{
    ImageViewer.Measure(new Size(dlg.PrintableAreaWidth, dlg.PrintableAreaHeight));
    ImageViewer.Arrange(new Rect(new Point(0, 0), ImageViewer1.DesiredSize));

    dlg.PrintVisual(ImageViewer, "Print a Large Image");
}

ImageViewer in my example could be replaced with any UIElement container such as a stackpanel, canvas, grid, ect. ImageViewer.Source should be set to the BitmapImage that is ready to be printed.

I got the idea from this page: http://www.switchonthecode.com/tutorials/printing-in-wpf