Image shows in the background after rotating

I don't think this is possible without extra code.

Graphics is associated with the image, so drawing will change the image.

So, you will need to have a 2nd image. Creating an empty one (or an exact copy) is not so slow..

Think of of it this way: Where could the old pixels come from when you are at the same time changing them? So you need to have two buffers. (But yes, internally, there is already a 2nd buffer or else the results would be even a lot weirder. But you can't control the use of it..)

If you really feel the need to avoid a 2nd image you can create a GraphicsPath or a Polygon that covers all but the rotated image and fill it with your background color..

But since rotating an image will need more space to accomodate the rotated corners you probably need a 2nd, larger image anyway..

Update: Here is an example of how to clear/crop the area outside the rotated imgae. It uses a GraphicsPath to which I first add a huge and then the target rectangle. This way a hole is cut and only the outside area is filled:

public static Image RotateImage(Image img, float rotationAngle)
{
    using (Graphics graphics = Graphics.FromImage(img))
    {
        graphics.TranslateTransform((float)img.Width / 2, (float)img.Height / 2);
        graphics.RotateTransform(rotationAngle);
        graphics.TranslateTransform(-(float)img.Width / 2, -(float)img.Height / 2);
        graphics.DrawImage(img, new Point(0, 0));

        GraphicsPath gp = new GraphicsPath();
        GraphicsUnit gu = GraphicsUnit.Pixel;
        gp.AddRectangle(graphics.ClipBounds);
        gp.AddRectangle(img.GetBounds(ref gu));
        graphics.FillPath(Brushes.White, gp);
    }
    return img;
}

Note that you can't use a transparent brush as GDI+ will not paint full transparency. Instead you would need to

1. Set the CompositingModefrom its default SourceOver to SourceCopy;
2. Fill with a very distinct color, not in your image, maybe Fuchsia
3. Use MakeTransparent.

graphics.CompositingMode = CompositingMode.SourceCopy;
..
graphics.FillPath(Brushes.Fuchsia, gp);
((Bitmap)img).MakeTransparent(Color.Fuchsia);

Note that not all applications will show transparency well.. Photoshop, of course, does..: