Are you prass the play button?

If you not to prass that.

Or if you want to preview but not prass the play button, you can add this to you script in top.

@script ExecuteInEditMode()

Try using this code. Make sure to set the Material to Unlit/Texture, if you use the normal Diffuse, this will not work.

using UnityEngine;
using System.Collections;

public class MinimapTest : MonoBehaviour
{

    public Material aMaterial;
    public Texture aTexture;
    void OnGUI ()
    {

       Graphics.DrawTexture(new Rect(100,100, 128, 128),aTexture, aMaterial);

    }
}