How To place a exactly Screen Sized Background in XNA /Windows Phone 7

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Mobile

Windows-Phone-7

XNA4.0

i am developing a game in Silverlight+XNA (integrated) platform for windows Phone 7. I Need Some Help in Loading a Full Screen Image (800X480) directly as the Background in XNA , i've tried many times but did not succeeded doing so , sometimes it goes on Square Shape (480X480) only , and sometimes it automatically do the stretching. i think its the problem of "DiplayOrientation" but that is already set to Portrait. and i am still unable to place the background exactly on the screen according to the screen size .

My background image size is 800X480 px , i've tried with 480X800 also .
Here are my Code Lines which are related to the Background .

Texture2D  backgroundTexture;

//in NavigatedTo() Method
 backgroundTexture = contentManager.Load<Texture2D>("2");// 2 is Image name

//in OnUpdate() Method
rectangleBackground = new Microsoft.Xna.Framework.Rectangle(0,0, backgroundTexture.Height, backgroundTexture.Width);
          
// and Finally in onDraw() Method
spriteBatch.Draw(backgroundTexture, rectangleBackground, Color.White);