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IsSelected property of WPF TabItem
<TabItem Name="tabItemGeneral">
    <TabItem.Triggers>
        <Trigger Property="IsSelected" Value="True">
            <Setter TargetName="tabGeneralImg" Property="Source" Value="Image/tabGeneralActive.png"></Setter>
        </Trigger>
    </TabItem.Triggers>
    <TabItem.Header>
        <StackPanel Orientation="Horizontal" Margin="0,-3,0,0">
            <Image Name="tabGeneralImg" Source="Images/tabGeneral.png" Width="11" Height="11"></Image>
            <Label Name="tabGeneralLbl" Content="General"></Label>
        </StackPanel>
    </TabItem.Header>
    <Grid>
        <!-- ... -->
    </Grid>
</TabItem>

<TabControl>
    <TabControl.Resources>
        <DataTemplate x:Key="tabItemGeneralHeaderTemplate">
            <StackPanel Orientation="Horizontal" Margin="0,-3,0,0">
                <Image Name="tabGeneralImg" Source="Image/tabGeneralActive.png" Width="11" Height="11"></Image>
                <Label Name="tabGeneralLbl" Content="General"></Label>
            </StackPanel>
            <DataTemplate.Triggers>
                <DataTrigger Binding="{Binding Path=IsSelected,RelativeSource={RelativeSource TemplatedParent}}" Value="True">
                    <Setter TargetName="tabGeneralImg" Property="Source" Value="Images/tabGeneral.png"/>
                </DataTrigger>
            </DataTemplate.Triggers>
        </DataTemplate>
    </TabControl.Resources>
    <TabItem Name="tabItemGeneral" HeaderTemplate="{StaticResource tabItemGeneralHeaderTemplate}">
        <Grid>
            ...
        </Grid>
    </TabItem>
</TabControl>

<Trigger Property="TabItem.IsSelected" Value="True">
  <Setter TargetName="tabGeneralImg" Property="Source" Value="Image/tabGeneralActive.png">
  </Setter>
</Trigger>