var icon : Texture;

function OnGUI () {
if(!icon) {
Debug.LogError("Add a texture on the inspector first");
return;
}
GUI.Button (Rect (0, 0, 100, 20), GUIContent (icon));
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public Texture icon;
void OnGUI() {
if (!icon) {
Debug.LogError("Add a texture on the inspector first");
return;
}
GUI.Button(new Rect(0, 0, 100, 20), new GUIContent(icon));
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public icon as Texture

def OnGUI():
if not icon:
Debug.LogError('Add a texture on the inspector first')
return
GUI.Button(Rect(0, 0, 100, 20), GUIContent(icon))