Example for using GUILayoutUtility to draw a texture

EditorGUILayoutExample.cs

using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(MyMono))]
public class EntityInspector : Editor {

    public override void OnInspectorGUI()
    {
        MyMono mono = (MyMono)target;

        DrawDefaultInspector();
        EditorGUILayout.LabelField("Item Icon: ");
        Rect rect = GUILayoutUtility.GetRect(100, 100);
        EditorGUI.DrawTextureTransparent(rect, mono.texture, ScaleMode.ScaleToFit);
    }
}