To sum up comments instead of changing Style
you need to move StackPanel
from DataTemplate
directly into HierarchicalDataTemplate
as at the moment used template has no content:
<HierarchicalDataTemplate x:Key="TreeTemplate" ItemsSource="{Binding Path=Children}">
<StackPanel Orientation="Horizontal">
<Image Name="img" ... />
<TextBlock Text="{Binding DisplayName, Mode=OneTime}" ... />
</StackPanel>
</HierarchicalDataTemplate>