Binding from DataContext in Blend 3

Question

i'm trying to build a simple application, that changes images based on Selected item in ListBox.

<Grid>
   <Grid>
      <Image />
   </Grid>
   <ListBox />
</Grid>

The XAML looks similar to this. So i set DataContext of the "inside" Grid to the ListBox's SelectedItem property. The problem is that when i try to DataBind Image source property to the one from DataContext, i am not seeing anything in the "Explicit Data Context" pane. Help please.