int imW = (int)scannedImage.ActualWidth; //scannedImage is my System.Windows.Controls.Image
int imH = (int)scannedImage.ActualHeight;
RenderTargetBitmap renderTargetBitmap = new RenderTargetBitmap(imW, imH, 100, 100, PixelFormats.Default);
renderTargetBitmap.Render(scannedImage); // Your image object
PngBitmapEncoder pngBitmapEncoder = new PngBitmapEncoder();
pngBitmapEncoder.Frames.Add(BitmapFrame.Create(renderTargetBitmap));
System.Drawing.Image im2 = //Here I want to convert to a System.Drawing.Image
makePDF("c:\\testPDF.pdf", im2); //A method I have that takes two arguments - path and System.Drawing.Image