Custom Icons

public class GizmoIconUtility
{
    [DidReloadScripts]
    static GizmoIconUtility()
    {
        EditorApplication.projectWindowItemOnGUI = ItemOnGUI;
    }

    static void ItemOnGUI(string guid, Rect rect)
    {
        string assetPath = AssetDatabase.GUIDToAssetPath(guid);

        FacePreset obj = AssetDatabase.LoadAssetAtPath(assetPath, typeof(FacePreset)) as FacePreset;

        if (obj != null)
        {
            rect.height = rect.width;
            GUI.DrawTexture(rect, (Texture2D)AssetDatabase.LoadAssetAtPath(Path.GetDirectoryName(assetPath) + "/Icons/" + obj.name + ".png", typeof(Texture2D)));
        }
    }
}