Capturing screenshots from games

[C# Programming]

Hi
I need to capture screenshots from DirectX games. I am using this code but sometimes I get totally blank images or the image of the desktop (but the game is open)

int screenWidth = Screen.GetBounds(new Point(0, 0)).Width;
int screenHeight = Screen.GetBounds(new Point(0, 0)).Height;
Bitmap bmpScreenShot = new Bitmap(screenWidth, screenHeight);
Graphics gfx = Graphics.FromImage((Image)bmpScreenShot);
gfx.CopyFromScreen(0, 0, 0, 0, new Size(screenWidth, screenHeight));
bmpScreenShot.Save("test.jpg", ImageFormat.Jpeg);

I really need help.

Thanks.