Visual Basic / WPF / 3D

I'm trying to place a texture in a 3d object, using Visual Basic and WPF, but I'm not being able to do it.
If instead of an image I render a blue square, by replacing the line  
mymodel3dgroup.Children.Add(New GeometryModel3D(mymesh, df))
for
mymodel3dgroup.Children.Add(New GeometryModel3D(mymesh, New DiffuseMaterial(Brushes.Blue)))
then I get a blue square. But what I need is to see the image, not just paint the object(s).
Can you please help? 
My thanks in advanced


    Imports System.Windows.Media.Media3D
    
    Public Class UserControl1
        Dim mymesh As New MeshGeometry3D
        Dim myviewport3d As New Viewport3D
        Dim mymodelvisual As New ModelVisual3D
        Dim mypccamera As New PerspectiveCamera(New Point3D(0, 0, -5), New Vector3D(0, 0, 1), New Vector3D(0, 1, 0), 100)
        Dim mymodel3dgroup As New Model3DGroup
        Dim myMaterial As DiffuseMaterial
    
        Private Sub UserControl1_Loaded(sender As Object, e As RoutedEventArgs) Handles Me.Loaded
    
            contentcontrol1.Content = myviewport3d
    
    
            myviewport3d.Children.Add(mymodelvisual)
    
            myviewport3d.Camera = mypccamera
    
            mymodel3dgroup.Children.Add(New DirectionalLight(Colors.White, New Vector3D(0, 0, 1)))
    
            mymodelvisual.Content = mymodel3dgroup
    
            mymesh.Positions.Add(New Point3D(0, 7, 0))
            mymesh.Positions.Add(New Point3D(7, 7, 0))
            mymesh.Positions.Add(New Point3D(7, 0, 0))
            mymesh.Positions.Add(New Point3D(7, 0, 0))
            mymesh.Positions.Add(New Point3D(0, 0, 0))
            mymesh.Positions.Add(New Point3D(0, 7, 0))
    
            Dim m As System.Windows.Media.Media3D.Material
            'Dim m = New System.Windows.Media.Media3D.Material()
            Dim df As System.Windows.Media.Media3D.DiffuseMaterial
            Dim spm As System.Windows.Media.Media3D.SpecularMaterial
    
            Dim ri As Uri = New Uri("d:\transferir.jpeg", UriKind.Absolute)
            Dim imgSource As ImageSource = New BitmapImage(ri)
    
           
    
            Dim ib = New ImageBrush()
            ib.ImageSource = New BitmapImage(New Uri("d:\cc.png", UriKind.Absolute))
            ib.Stretch = Stretch.Fill
            ib.ViewboxUnits = BrushMappingMode.Absolute
            ib.AlignmentX = AlignmentX.Left
            ib.AlignmentY = AlignmentY.Top
            
    
            ib.Opacity = 0.5
    
    
            df = New System.Windows.Media.Media3D.DiffuseMaterial(ib)
    
            spm = New System.Windows.Media.Media3D.SpecularMaterial(ib, 85)
           
            mymodel3dgroup.Children.Add(New GeometryModel3D(mymesh, df))
            
    
    
        End Sub
    
       
    End Class