BackgroundImageListBox.xaml

<ListBox>
    <ListBox.Template>
        <ControlTemplate TargetType="{x:Type ListBox}">
            <Border x:Name="Bd" SnapsToDevicePixels="true"
                    Background="{TemplateBinding Background}"
                    BorderBrush="{TemplateBinding BorderBrush}"
                    BorderThickness="{TemplateBinding BorderThickness}"
                    Padding="1">
                <Grid>
                    <Image HorizontalAlignment="Right"
                           VerticalAlignment="Bottom"
                           Stretch="None"
                           Source="RSS.png"
                           Margin="0,0,15,15"
                           Opacity="0.25">
                    </Image>
                    <ScrollViewer Padding="{TemplateBinding Padding}"
                                  Focusable="false">
                        <ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                    </ScrollViewer>
                </Grid>
            </Border>
            <ControlTemplate.Triggers>
                <Trigger Property="IsEnabled" Value="false">
                    <Setter Property="Background"
                            TargetName="Bd"
                            Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
                </Trigger>
                <Trigger Property="IsGrouping" Value="true">
                    <Setter Property="ScrollViewer.CanContentScroll"
                            Value="false"/>
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>
    </ListBox.Template>
</ListBox>