When I show a picture on the GUI there is a square behind the picture

I have added a bar to my game, but behind it there is an strange square, here you have a pic: http://imageshack.us/photo/my-images/263/sinttulord.png/ What I’m doing wrong?

Here you have the script:

GUI.BeginGroup (new Rect (pos.x, pos.y, size.x, size.y));
    GUI.Box (Rect (0,0, size.x, size.y),progressBarEmpty);
GUI.EndGroup ();

Hope you could help me, lot of thanks

That is what GUI.Box does.

Use Label (it will show texture only):

GUI.Label (Rect (0,0, size.x, size.y),progressBarEmpty);

source: http://unity3d.com/support/documentation/ScriptReference/GUI.Label.html