Problem storing image matrix

If your image is monochrome you may try something like:

int[,] BitmapToMatrix(Bitmap bmp)
{
    Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
    BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.WriteOnly,
        bmp.PixelFormat);
    int bytesPerPixel = bmpData.Stride / bmp.Width;
    //if you want to access your pixel as matrix[i, j]
    //(i = row index, j = column index)
    //then allocate your matrix like this:
    int[,] matrix = new int[bmp.Height, bmp.Width];
    //we will use pointers
    unsafe
    {
        for (int i = 0; i < bmp.Height; i++)
        {
            byte* p = (byte*)bmpData.Scan0.ToPointer() +
                (i * bmp.Height) * bmpData.Stride;
            for (int j = 0; j < bmp.Width; j++)
            {
                //don't bother with color components,
                //just extract the first pixel value
                byte* pixel = p + j * bytesPerPixel;
                matrix[i, j] = (int)*pixel;
            }
        }
    }
    bmp.UnlockBits(bmpData);
    return matrix;
}

If you don't want to use unsafe code, then you may have a look to:
http://msdn.microsoft.com/en-us/library/system.drawing.bitmap.unlockbits.aspx

If you want to store the colors, then try something like:

Color[,] BitmapToMatrix(Bitmap bmp)
{
    //if you want to access your pixel as matrix[i, j]
    // (i = row index, j = column index)
    //then allocate your matrix like this:
    Color[,] matrix = new Color[bmp.Height, bmp.Width];
    for (int i = 0; i < bmp.Height; i++)
    {
        for (int j = 0; j < bmp.Width; j++)
        {
           //This work for any bitmap format but is very slow
           //if you want to be more efficient,
           //consider using the first solution.
           //But you must then deal with the internal data
           //by yourself to extract the color components
            matrix[i, j] = bmp.GetPixel(j, i);
        }
    }

    return matrix;
}