Add an Icon property to allow displaying images in the RadPanelBarItem Header.

Declined:

Adding such property requires also adding a visual element (Image) in the Template of the RadPanelBarItems.
RadTreeView also has Image in its ControlTemplate and we have to admit this was our design mistake in the past. Such element makes the Style larger, makes the virtualization harder to support, slows down the performance.

How To Add Image in RadPanelBar:

In non-databound scenarios just add the Image in the Header:

      <telerik:RadPanelBarItem>
                <telerik:RadPanelBarItem.Header>
                    <StackPanel>
                        <Image Source="MyImage.png" Margin="5"/>
                        <TextBlock Text="{Binding Name}" />
                    </StackPanel>
                </telerik:RadPanelBarItem.Header>
            </telerik:RadPanelBarItem>

In databound scenario set the ItemTemplate of the RadPanelBar:

DataTemplate x:Key="itemTemplate">
                <StackPanel>
                    <Image Source="MyImage.png" Margin="5"/>
                    <TextBlock Text="{Binding Name}" />
                </StackPanel>
            </DataTemplate>

 <telerik:RadPanelBar ItemTemplate="{StaticResource itemTemplate}"/>

See the SDK example demonstrating those approaches: https://github.com/telerik/xaml-sdk/tree/master/PanelBar/AddHeaderIcon