You would have to create four bitmaps of the appropriate size, then draw the existing bitmap into them:
using (Image large = Bitmap.FromFile(@"D:\Test data\BigImage.jpg"))
{
using (Image small = new Bitmap(large.Width / 2, large.Height / 2))
{
using (Graphics g = Graphics.FromImage(small))
{
g.DrawImage(large, 0.0f, 0.0f);
small.Save(@"D:\Test Data\TopLeft.jpg", ImageFormat.Jpeg);
}
}
}