Cross axis GUI

I know that this is basically a n00b question but I need to create a cross axis(which is an image) in a GUI and so it would appear/disappear when I turn on something (e.g. pause menu, cutscene start…) again, n00b question but I have no idea how to implement an image into a GUI

you usually use GUI.DrawTexture:

(in c#)
(Dont Mind the long sentence, it is actually quite simple. For more info: here)

//Assign the texture in the inspector.
public Texture2D myTexture;

//Draw a crosshair in the middle of the screen.
void OnGUI ()
{
GUI.DrawTexture(new Rect(Screen.width / 2 - myTexture.width / 2, Screen.height / 2 - myTexture.height / 2, myTexture.width, myTexture.height);
}

I hope this helps.