NotImplementedException in Graphics.DrawImage

It is a bit weird to notice that a .Net Framework method returns NotImplementedException. At least it was for me, mainly because it is not documented anywhere in the msdn documentation.

It turns out that Graphics.DrawImage throws this exception when an Image object is passed in. This is really funny, since this method actually expects an Image object as its first argument, so my first-try code to print an image file was:

public void PrintImage(string fileName) {
    Image img = Image.FromFile(fileName);
    PrintDocument printDoc = new PrintDocument();
    printDoc.DocumentName = this.m_sFilename;
    printDoc.PrintPage += new PrintPageEventHandler(PrintPage);
    printDoc.Print();
}

private void PrintPage(object sender, PrintPageEventArgs e) {
     // Throws NotImplementedException!!!
    e.Graphics.DrawImage(this.printedImage,
        new Point(e.MarginBounds.Left, e.MarginBounds.Top));
    e.HasMorePages = false;
}

Weird, eh?! So, the solution?

Bitmap img = (Bitmap)Image.FromFile(fileName);

Bitmap class extends Image, so, IMHO, DrawImage should determine in runtime that the image passed in is actually is a Bitmap and do whatever it has to do, but, strangely, it complains and throws a NotImplementedException for the exactly same runtime object.

Can anyone explain what is happening here?