Hi Adam,
I'm sorry but no, the Image isn't part of the template for that usage scenario any more. You could make a custom template that has the Image in it and apply that to the buttons. Here's the template that gets applied right now. You could put an image next to the TextBlock in a clone of this an reference the updated template for your instances.
<ControlTemplate x:Key="BackstageHeaderButtonTemplate" TargetType="{x:Type ribbonControlsPrimitives:ButtonBase}"><Border Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" SnapsToDevicePixels="True"><Border x:Name="innerBorder" BorderThickness="{TemplateBinding BorderThickness}" SnapsToDevicePixels="True"><shared:PixelSnapper Margin="{TemplateBinding Padding}"><TextBlock x:Name="label" FontSize="13" Text="{TemplateBinding Label}" Foreground="{TemplateBinding Foreground}" VerticalAlignment="Center" SnapsToDevicePixels="True" /></shared:PixelSnapper></Border></Border><ControlTemplate.Triggers><Trigger Property="IsMouseOver" Value="True"><Setter Property="Background" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemBackgroundHoverBrushKey}}" /><Setter Property="BorderBrush" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemBorderHoverBrushKey}}" /><Setter TargetName="innerBorder" Property="BorderBrush" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemInnerBorderHoverBrushKey}}" /><Setter TargetName="label" Property="TextElement.Foreground" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemForegroundHoverBrushKey}}" /></Trigger><Trigger Property="IsPressed" Value="True"><Setter Property="Background" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemBackgroundSelectedBrushKey}}" /><Setter Property="BorderBrush" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemBorderSelectedBrushKey}}" /><Setter TargetName="innerBorder" Property="Background" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemHighlightSelectedBrushKey}}" /><Setter TargetName="innerBorder" Property="BorderBrush" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemInnerBorderSelectedBrushKey}}" /><Setter TargetName="label" Property="TextElement.Foreground" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemForegroundSelectedBrushKey}}" /></Trigger><Trigger Property="IsKeyboardFocused" Value="True"><Setter Property="Background" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemBackgroundSelectedBrushKey}}" /><Setter Property="BorderBrush" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemBorderSelectedBrushKey}}" /><Setter TargetName="innerBorder" Property="Background" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemHighlightSelectedBrushKey}}" /><Setter TargetName="innerBorder" Property="BorderBrush" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemInnerBorderSelectedBrushKey}}" /><Setter TargetName="label" Property="TextElement.Foreground" Value="{DynamicResource {x:Static themes:AssetResourceKeys.BackstageTabItemForegroundSelectedBrushKey}}" /></Trigger><Trigger Property="IsEnabled" Value="False"><Setter TargetName="label" Property="Opacity" Value="0.4"/></Trigger></ControlTemplate.Triggers></ControlTemplate>