Convert an image to grayscale

Bitmap d = new Bitmap(c.Width, c.Height);

for (int i = 0; i < c.Width; i++)
{
    for (int x = 0; x < c.Height; x++)
    {
        Color oc = c.GetPixel(i, x);
        int grayScale = (int)((oc.R * 0.3) + (oc.G * 0.59) + (oc.B * 0.11));
        Color nc = Color.FromArgb(oc.A, grayScale, grayScale, grayScale);
        d.SetPixel(i, x, nc);
    }
}

This way it also keeps the alpha channel.
Enjoy.

There's a static method in ToolStripRenderer class, named CreateDisabledImage. Its usage is as simple as:

Bitmap c = new Bitmap("filename");
Image d = ToolStripRenderer.CreateDisabledImage(c);

It uses a little bit different matrix than the one in the accepted answer and additionally multiplies it by a transparency of value 0.7, so the effect is slightly different than just grayscale, but if you want to just get your image grayed, it's the simplest and best solution.

"I want a Bitmap d, that is grayscale. I do see a consructor that includes System.Drawing.Imaging.PixelFormat, but I don't understand how to use that."

Here is how to do this

Bitmap grayScaleBP = new 
         System.Drawing.Bitmap(2, 2, System.Drawing.Imaging.PixelFormat.Format16bppGrayScale);

EDIT: To convert to grayscale

             Bitmap c = new Bitmap("fromFile");
             Bitmap d;
             int x, y;

             // Loop through the images pixels to reset color.
             for (x = 0; x < c.Width; x++)
             {
                 for (y = 0; y < c.Height; y++)
                 {
                     Color pixelColor = c.GetPixel(x, y);
                     Color newColor = Color.FromArgb(pixelColor.R, 0, 0);
                     c.SetPixel(x, y, newColor); // Now greyscale
                 }
             }
            d = c;   // d is grayscale version of c  

Faster Version from switchonthecode follow link for full analysis:

public static Bitmap MakeGrayscale3(Bitmap original)
{
   //create a blank bitmap the same size as original
   Bitmap newBitmap = new Bitmap(original.Width, original.Height);

   //get a graphics object from the new image
   using(Graphics g = Graphics.FromImage(newBitmap)){

       //create the grayscale ColorMatrix
       ColorMatrix colorMatrix = new ColorMatrix(
          new float[][] 
          {
             new float[] {.3f, .3f, .3f, 0, 0},
             new float[] {.59f, .59f, .59f, 0, 0},
             new float[] {.11f, .11f, .11f, 0, 0},
             new float[] {0, 0, 0, 1, 0},
             new float[] {0, 0, 0, 0, 1}
          });

       //create some image attributes
       using(ImageAttributes attributes = new ImageAttributes()){

           //set the color matrix attribute
           attributes.SetColorMatrix(colorMatrix);

           //draw the original image on the new image
           //using the grayscale color matrix
           g.DrawImage(original, new Rectangle(0, 0, original.Width, original.Height),
                       0, 0, original.Width, original.Height, GraphicsUnit.Pixel, attributes);
       }
   }
   return newBitmap;
}