combox selected index didnt work on picturebox

Now what i was doing is when i click a button group box 1 and group box 2 will show out and i have diffrent button will change the combobox value.

Private Sub Button4_Click(sender As Object, e As EventArgs) Handles Button4.Click



    GroupBox1.Show()
    GroupBox2.Show()

    comboboxselectdiffrent.Items.Add("pizza_ChickenSupreme")
    comboboxselectdiffrent.Items.Add("pizza_CockadoodleBacon")

    If comboboxselectdiffrent.SelectedIndex = 0 Then
        PictureBox1.Image = PIZZA_HUT_SYSTEM_NEW_VER.My.Resources.Resources.pizza_ChickenSupreme

    ElseIf comboboxselectdiffrent.SelectedIndex = 1 Then
        PictureBox1.Image = PIZZA_HUT_SYSTEM_NEW_VER.My.Resources.Resources.pizza_CockadoodleBacon

    End If

    End Sub

Can anyone tell me what i was do wrong? i have no idea why it wont work