GUIContent.image  

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))