You need to add the following in the DockingManager XAML:
<avalonDock:DockingManager.AnchorableHeaderTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<Image Source="{Binding IconSource}" Margin="0,0,4,0"/>
<TextBlock Text="{Binding Title}" />
</StackPanel>
</DataTemplate>
</avalonDock:DockingManager.AnchorableHeaderTemplate>
