Your best option is to manage the toggle button path / image in xaml by defining a new style, the below code demonstrates this.  You will want to tweak this to your own needs of course.

<Style x:Key="ToggleStyleZoom" TargetType="ToggleButton">
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ToggleButton">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates" ic:ExtendedVisualStateManager.UseFluidLayout="True">
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames BeginTime="00:00:00" Duration="00:00:00.0010000" Storyboard.TargetName="Bd" Storyboard.TargetProperty="(Border.Background)">
<DiscreteObjectKeyFrame KeyTime="00:00:00" Value="{StaticResource btnHoverBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames BeginTime="00:00:00" Duration="00:00:00.0010000" Storyboard.TargetName="Bd" Storyboard.TargetProperty="(Border.Background)">
<DiscreteObjectKeyFrame KeyTime="00:00:00" Value="{StaticResource btnPressedBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled"/>
</VisualStateGroup>
<VisualStateGroup x:Name="CheckStates">
<VisualState x:Name="Checked"/>
<VisualState x:Name="Unchecked"/>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Focused"/>
<VisualState x:Name="Unfocused"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<VisualStateManager.CustomVisualStateManager>
<ic:ExtendedVisualStateManager/>
</VisualStateManager.CustomVisualStateManager>
<Border x:Name="Bd" Background="{StaticResource BaseBorderBrushKey}" BorderBrush="#BFFFFFFF" BorderThickness="1,0,1,1" CornerRadius="0,0,2,2">
<Border Margin="1,0,1,1" BorderBrush="#59FFFFFF" BorderThickness="1,0,1,1" CornerRadius="0,0,1,1">
<Grid>
<Image Margin="51,8,52,8" Source="Images/zoom_16.png" Stretch="Fill"/>
</Grid>
</Border>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="Background" Value="{StaticResource BaseBorderBrushKey}"/>
<Setter Property="Foreground" Value="White"/>
</Style>