The texture is not following the mouse. It follows the mouse horizontlally but vertically it goes up when I move the mouse down and oppossite. They meet in the middle of the screen.
You shouldn't use Input.mousePosition in OnGUI...use Event.current.mousePosition instead. It's easier and faster; you don't need to mess with Screen.height:
var crosshairTexture : Texture2D;
function OnGUI () {
var mousePos = Event.current.mousePosition;
GUI.DrawTexture( Rect( mousePos.x - (crosshairTexture.width/2),
mousePos.y - (crosshairTexture.height/2),
crosshairTexture.width,
crosshairTexture.height), crosshairTexture);
}
...works also ok in standalone. Only in webplayer in fullscreen mode the crosshair texture does not follow the mouse. mouse is pointing to the bottom of the texture. I've copied your script exactly.