Viewbox does not work in Listbox

Hi all,

Please see my code as below.

The left Viewbox in Grid works well. But the right Viewbox in listbox does not work.

What I want is that when resize window and decrease the height of window, then the ico should become smaller and smaller.

so, how do I make the Viewbox in listbox work?

<GridWidth="200"HorizontalAlignment="Left">
            <ViewboxStretch="Uniform"MaxHeight="68">
               <ImageSource="MyIco.ico"MaxHeight="68"/>
            </Viewbox>
        </Grid>
        <ListBoxMargin="200,0,0,0"ItemsSource="{Binding Students}"ScrollViewer.HorizontalScrollBarVisibility="Disabled"ScrollViewer.VerticalScrollBarVisibility="Disabled"VerticalContentAlignment="Stretch">
            <ListBox.ItemTemplate>
               <DataTemplate>
                   <ViewboxStretch="Uniform"MaxHeight="68">
                       <ImageSource="MyIco.ico"MaxHeight="68"/>
                   </Viewbox>
               </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>