Using Background Images in the RepeatButton of a ScrollBarViewer doesn't show the image

I am trying to create my own scrollviewer in WPF and it is working fine except for background images in the RepeatButton. If I use a SolidColorBrush instead of an ImageBrush it works fine. Below is my xaml and I have verified the ImageSource can be found by adding it somewhere else in the scrollbar (specifically the track image). Do you know why this is? The forground Image shows fine.

<Style x:Key="ScrollBarButtonStyle" TargetType="RepeatButton">
    <Setter Property="HorizontalContentAlignment" Value="Center"/>
    <Setter Property="VerticalContentAlignment" Value="Center"/>
    <Setter Property="Width" Value="50"/>
    <Setter Property="Height" Value="50"/>        
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="RepeatButton">
                <Grid Name="ButtonBackground">
                    <VisualStateManager.VisualStateGroups>
                        <VisualStateGroup x:Name="CommonStates">
                            <VisualState x:Name="Normal">
                                <Storyboard>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ButtonBackground" Storyboard.TargetProperty="Background">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <ImageBrush ImageSource="{DynamicResource ListItemBackgroundBttnNormal}"/>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Pressed">
                                <Storyboard>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ButtonBackground" Storyboard.TargetProperty="Background">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <ImageBrush ImageSource ="{DynamicResource ListItemBackgroundBttnPressed}"/>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Disabled">
                                <Storyboard>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ButtonBackground" Storyboard.TargetProperty="Background">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <ImageBrush ImageSource = "{DynamicResource ListItemBackgroundBttnDisabled}"/>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                    <DoubleAnimation Storyboard.TargetName="contentPresenter" Storyboard.TargetProperty="Opacity" From="1.0" To="0.5"/>
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="MouseOver">
                                <Storyboard>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ButtonBackground" Storyboard.TargetProperty="Background">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <ImageBrush ImageSource="{DynamicResource ListItemBackgroundBttnNormal}"/>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                </Storyboard>
                            </VisualState>
                        </VisualStateGroup>
                    </VisualStateManager.VisualStateGroups>
                    <ContentPresenter
                          x:Name="contentPresenter"
                          Content="{TemplateBinding Content}"
                          ContentTemplate="{TemplateBinding ContentTemplate}"
                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                          Margin="{TemplateBinding Padding}"/>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

Try to change the DynamicResource to StaticResource and see if it still works. The DynamicResource doesn't show any error if the path is not found, where the StaticResource does tihs. I assume that your image sources are not known in this context. Make sure that you add your Resource in the App.xaml at <ResourceDictionary.MergedDictionaries>.