That helped so much, thank you!  I had to  play around bit with the number of column  and   row definition  entries,  but it now properly fits  the marquee.  My XML looks like this now:
 
	 
	<UserControl xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
	             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
	             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
	             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
	             xmlns:transitions="clr-namespace:Unbroken.LaunchBox.Wpf.Transitions;assembly=Unbroken.LaunchBox.Wpf" 
	             xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity" 
	             xmlns:cal="http://www.caliburnproject.org" 
	             mc:Ignorable="d" 
	             d:DesignHeight="562" d:DesignWidth="1000" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Style="{DynamicResource UserControlStyle}"> 
	<!-- GRID DEFINITIONS --> 
	             
	             
	    <Grid Height="1080" Width="1920"> 
	        <Grid.ColumnDefinitions>                        
	            <ColumnDefinition Width="1*" /> 
	    </Grid.ColumnDefinitions> 
	        <Grid.RowDefinitions> 
	            <RowDefinition Height="*" /> 
	            <RowDefinition Height="*" /> 
	            <RowDefinition Height="*" /> 
	        </Grid.RowDefinitions> 
	                
	                
	        <!-- GAME MARQUEE --> 
	                
	                
	        <Image Source="{Binding Path=SelectedGame.MarqueeImagePath}" Stretch="fill" HorizontalAlignment="Center" VerticalAlignment="Top" RenderOptions.BitmapScalingMode="HighQuality" /> 
	            <TextBlock x:Name="GameMarquee" Visibility="Visible"> 
	                <TextBlock.Text> 
	                    <MultiBinding StringFormat="{}pack://siteoforigin:,,,/Arcade - Marquee/_Default.png"> 
	                    <Binding Path="SelectedGame.Platform"/> 
	                    </MultiBinding> 
	                </TextBlock.Text> 
	            </TextBlock> 
	        <Image x:Name="MainMarquee" Grid.RowSpan="3" Grid.Row="0" Grid.ColumnSpan="8" Grid.Column="0" Source="{Binding Text, ElementName=GameMarquee, FallbackValue='pack://siteoforigin:,,,/Arcade - Marquee/_Default.png'}" Opacity="100" Stretch="fill" Panel.ZIndex="1" RenderOptions.BitmapScalingMode="HighQuality" /> 
	    </Grid> 
	</UserControl>