Convert table double [,]into bmp 8 bpp

You could find an answer with some searching on Google. However, try reading this[^] to get some ideas.

[Edit added some more details]
Here is some code to create the bitmap

//Create the new bitmap
int w = 20;//use the desired height/width
int h = 20;
Bitmap newBmp = new Bitmap(w, h, System.Drawing.Imaging.PixelFormat.Format8bppIndexed);

//Build a grayscale color Palette
ColorPalette grayscalePalette = newBmp.Palette;
for (int i = 0; i < 256; i++)
{
    Color tmp = Color.FromArgb(255, i, i, i);
    grayscalePalette.Entries[i] = Color.FromArgb(255, i, i, i);
}
newBmp.Palette = grayscalePalette;

//now add the data as the link above shows

By the way, a data range of 0-255 is 8 bits per pixel.