DitherType
Specifies how images are dithered with a reduced color palette.
                
DualSpiral4x4 = 7
Dithering is performed using the colors in one of the standard fixed palettes.
DualSpiral8x8 = 8
Dithering is performed using the colors in one of the standard fixed palettes.
ErrorDiffusion = 9
Dithering is performed based on the palette specified by the palette parameter of the
           ConvertFormat method.
          This algorithm can be used with any palette.
None = 0
No dithering is performed. Pixels in the source bitmap are mapped to the nearest color in the palette specified
          by the palette parameter of the  ConvertFormat
          method. This algorithm can be used with any palette other than  Custom.
Ordered16x16 = 4
Dithering is performed using the colors in one of the standard fixed palettes.
Ordered4x4 = 2
Algorithm to perform dithering based on the colors in one of the standard fixed palettes. 
          This algorithm can also be used to convert a bitmap to a 16-bits-per-pixel format that has no palette.
Ordered8x8 = 3
Dithering is performed using the colors in one of the standard fixed palettes.
Solid = 1
No dithering is performed. Pixels in the source bitmap are mapped to the nearest color in the palette specified
          by the palette parameter of the  ConvertFormat
          method. This algorithm can be used with any palette.
Spiral4x4 = 5
Dithering is performed using the colors in one of the standard fixed palettes.
Spiral8x8 = 6
Dithering is performed using the colors in one of the standard fixed palettes.