The icon image contained.

	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

public class Example(MonoBehaviour):

	public icon as Texture

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