Create custom icon from assets

[UrbanSide]

Hello,how i can change icon to custom image?
For example in:

                <ui:NavigationItem x:Name="Server"
                    Content="{x:Static properties:Language.MainWindow_Server}"
                    Icon="ServerPlay20" FontSize="10" PageType="{x:Type Pages:Page_Server}" ClickMode="Press"
                    />
                <ui:NavigationSeparator />

I want change icon to assets/server.png

[UrbanSide]

I haven't found a solution yet other than

                 <ui:NavigationItem IsActive="False" IconSize="48" HorizontalAlignment="Center" Margin="0,0,0,4">
                     <ui:NavigationItem.Image>
                         <BitmapImage UriSource="/emblem.png" Rotation="Rotate0" CreateOptions="None"></BitmapImage>
                     </ui:NavigationItem.Image>
                 </ui:NavigationItem>

But with it, when active=false the icon jumps up