C# Graphics error when creating a rectangle on top of a tif

What I have is a .tif file being loaded into a picture box. When I try to draw a rectangle on top of that tif it throws the following error:

A Graphics object cannot be created from an image that has an indexed pixel for开发者_StackOverflowmat.

here is the code i'm using, it throws the error at using (Graphics g = Graphics.FromImage(img)). img is the file being loaded into the picture box.

                           using (Graphics g = Graphics.FromImage(img))                            
                        {
                            int x1value = Convert.ToInt32(x1);
                            int y1value = Convert.ToInt32(y1);
                            int x3value = Convert.ToInt32(x3);
                            int y3value = Convert.ToInt32(y3);
                            SolidBrush blackBrush = new SolidBrush(Color.Black);
                            g.FillRectangle(blackBrush, x1value, y1value, x3value, y3value);  

Image img = Image.FromFile(string2image);   
    Bitmap source = (Bitmap)img;   
    source = new Bitmap(320, 240,  System.Drawing.Imaging.PixelFormat.Format24bppRgb);   
    pictureBox1.Image = source; 
    pictureBox1.SizeMode = PictureBoxSizeMode.StretchImage; 

using (Graphics g = Graphics.FromImage(pictureBox1.Image))

This method actually only draws a gaint black box. not causes a blackbox to be written over the original file.

You're trying to get a graphics context from an image. You need to get one from a Bitmap buffer i believe.