Background image & WHIP

I've found a way to put an image as the background:

[CommandMethod("BG")]
public void Bg()
{
    Document doc = Application.DocumentManager.MdiActiveDocument;
    Database db = doc.Database;
    using (Transaction tr = db.TransactionManager.StartTransaction())
    {
        var imgBg = new ImageBackground
        {
            ImageFileName = @"...",
            FitToScreen = true
        };
        var bgDict = (DBDictionary)tr.GetObject(Background.GetBackgroundDictionaryId(db, true), OpenMode.ForWrite);
        bgDict.SetAt(Path.GetFileNameWithoutExtension(imgBg.ImageFileName), imgBg);
        tr.AddNewlyCreatedDBObject(imgBg, true);
        var vt = (ViewportTable)tr.GetObject(db.ViewportTableId, OpenMode.ForRead);
        var vtr = (ViewportTableRecord)tr.GetObject(vt[SymbolUtilityServices.ViewportActiveName], 
            OpenMode.ForWrite);
        vtr.Background = imgBg.Id;
        tr.Commit();
    }
}

 It works with AGS (3D visual style), but not with WHIP (2D Wireframe visual style). The image is not displayed.

Someone know how to make this work with WHIP?