How to put image "placemarkers" inside a scrollbar in WPF?

Short answer is "change the template of the ScrollBar".

Long answer is... That I would add an ItemsControl in the template of the ScrollBar control. I would put this ItemsControl on top of the template with its IsHitTestVisible set to false so that it does not capture mouse events.

Then I would use a Canvas as the ItemsPanelTemplate in order to be able to place the spots properly. I would use databinding with the ItemsSource property of the ItemsControl and a DataTemplate in order to render each element with an image.

Here is a sample that I did using Blend. Of course it's not complete (it does not handle mouse event for example), but I hope it will be a starting point for you.

_{(source: japf.fr)}

<ControlTemplate TargetType="{x:Type ScrollBar}">
    <Grid SnapsToDevicePixels="true" Background="{TemplateBinding Background}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition MaxWidth="{DynamicResource {x:Static SystemParameters.HorizontalScrollBarButtonWidthKey}}"/>
            <ColumnDefinition Width="0.00001*"/>
            <ColumnDefinition MaxWidth="{DynamicResource {x:Static SystemParameters.HorizontalScrollBarButtonWidthKey}}"/>
        </Grid.ColumnDefinitions>
        <RepeatButton Style="{StaticResource ScrollBarButton}" Command="{x:Static ScrollBar.LineLeftCommand}" Microsoft_Windows_Themes:ScrollChrome.ScrollGlyph="LeftArrow"/>
        <Track x:Name="PART_Track" Grid.Column="1" d:IsHidden="True">
            <Track.Thumb>
                <Thumb Style="{StaticResource ScrollBarThumb}" Microsoft_Windows_Themes:ScrollChrome.ScrollGlyph="HorizontalGripper"/>
            </Track.Thumb>
            <Track.IncreaseRepeatButton>
                <RepeatButton Style="{StaticResource HorizontalScrollBarPageButton}" Command="{x:Static ScrollBar.PageRightCommand}"/>
            </Track.IncreaseRepeatButton>
            <Track.DecreaseRepeatButton>
                <RepeatButton Style="{StaticResource HorizontalScrollBarPageButton}" Command="{x:Static ScrollBar.PageLeftCommand}"/>
            </Track.DecreaseRepeatButton>
        </Track>
        <ItemsControl Grid.Column="1" HorizontalAlignment="Stretch">
            <sys:Double>10</sys:Double>
            <sys:Double>50</sys:Double>
            <sys:Double>100</sys:Double>
            <sys:Double>140</sys:Double>
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Rectangle Fill="Orange" Width="3" Height="16"/>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
            <ItemsControl.ItemContainerStyle>
                <Style TargetType="ContentPresenter">
                    <Setter Property="Canvas.Left" Value="{Binding }" />
                </Style>
                                        </ItemsControl.ItemContainerStyle>
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <Canvas/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
        </ItemsControl>
        <RepeatButton Style="{StaticResource ScrollBarButton}" Grid.Column="2" Command="{x:Static ScrollBar.LineRightCommand}" Microsoft_Windows_Themes:ScrollChrome.ScrollGlyph="RightArrow" d:IsHidden="True"/>
    </Grid>
</ControlTemplate>