protected override void LoadGraphicsContent(bool loadAllContent)
{
   if (loadAllContent)
   {
      myTexture = content.Load<Texture2D>("globe");
      spriteBatch = new SpriteBatch(graphics.GraphicsDevice);                
   }           
}

protected override void Draw(GameTime gameTime)
{
   graphics.GraphicsDevice.Clear(Color.CornflowerBlue);

   spriteBatch.Begin(SpriteBlendMode.AlphaBlend);
   spriteBatch.Draw(myTexture, spritePosition, Color.White);
   spriteBatch.End();     
   
   base.Draw(gameTime);
}

private void Screenshot()
{
   using (Texture2D screenshot = new Texture2D(graphics.GraphicsDevice,                                                            
          graphics.GraphicsDevice.PresentationParameters.BackBufferWidth,                                                 
          graphics.GraphicsDevice.PresentationParameters.BackBufferHeight, 1,
                                                ResourceUsage.ResolveTarget,
                                                SurfaceFormat.Color,
                                                ResourceManagementMode.Manual))
            {                
                graphics.GraphicsDevice.ResolveBackBuffer(screenshot);
                screenshot.Save("test.bmp", ImageFileFormat.Bmp);
            }
}

I think probably the easiest thing to do would be to use a bool variable to track whether the UI should be displayed, then perform the following steps:

1. From within your Screenshot() method, set the UI bool variable to false.
2. Call your game's Draw() method, drawing the scene without the UI.
3. Take the screenshot.
4. Set the UI variable back to true and call Draw() again.

That's a good thought, and it might be better to execute the Screenshot() call from inside the Draw() method anyway (after drawing the scene but before drawing the UI) because after thinking about it, I'm not sure if it would work to manually call Draw() because the Game class may be setting various render state information before it automatically calls Draw(), and if you manually call Draw(), this render state information would not be set.