system.drawing.bitmap b = new System.Drawing.Bitmap("c:\original_image.gif");  

System.Drawing.Image thmbnail = b.GetThumbnailImage(100,75,null,new IntPtr()); 

OctreeQuantizer quantizer = new OctreeQuantizer ( 255 , 8 ) ; 

using ( Bitmap quantized = quantizer.Quantize ( thmbnail ) ) 

{ 

     quantized.Save("c:\thumnail.gif", System.Drawing.Imaging.ImageFormat.Gif); 

} 

octreequantizer grayquantizer = new GrayscaleQuantizer ( ) ; 

using ( Bitmap quantized = grayquantizer.Quantize ( thmbnail ) ) 

{ 

     quantized.Save("c:\thumnail.gif", System.Drawing.Imaging.ImageFormat.Gif); 

}