title: image color conversion
body: <p>I need to convert 24bppRGB to 16bppRGB, 8bppRGB, 4bppRGB, 8bpp grayscal and 4bpp grayscale. Any good link or other suggestions?</p>
  
  <p>preferably using Windows/GDI+</p>
  
  <p>[EDIT] speed is more critical than quality. source images are screenshots
  [EDIT1] color conversion is required to minimize space</p>