Setting ImageSource From Style Resource on UserControl - wpf

Question

I have created a User Control that has an image component inside bound to a DependencyProperty. Everything works great at runtime, but I can not see the image in Design Time.
ButtonBase.xaml:
<UserControl x:Class="VirtualEnvelopes.ButtonBase"
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:local="clr-namespace:VirtualEnvelopes"
x:Name="ButtonRoot">
<UserControl.Resources>
<BitmapImage x:Key="DefaultImage" UriSource="/assets/common/nullButton.png" />
</UserControl.Resources>
<Grid x:Name="LayoutRoot">
<Grid>
<Image x:Name="MainImageBitmap" Source="{Binding Path=MainImage, ElementName=ButtonRoot, TargetNullValue={StaticResource DefaultImage}}" Tag="{Binding Path=Tag, ElementName=ButtonRoot}" />
</Grid>
</Grid>
</UserControl>
ButtonBase.xaml.cs
public partial class ButtonBase : UserControl
{
public static readonly DependencyProperty MainImageProperty =
DependencyProperty.Register("MainImage", typeof(Uri), typeof(ButtonBase), new UIPropertyMetadata(null));
public Uri MainImage
{
get { return (Uri)GetValue(MainImageProperty); }
set { SetValue(MainImageProperty, value); }
}
public ButtonBase()
{
InitializeComponent();
}
}
Usage:
<Window x:Class="VirtualEnvelopes.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:base="clr-namespace:VirtualEnvelopes">
<Window.Resources>
<Style x:Key="Button1Style" TargetType="{x:Type base:ButtonBase}">
<Setter Property="MainImage" Value="pack://application:,,,/pathToImage.png" />
</Style>
</Window.Resources>
<Canvas Width="1000" Height="1000">
<base:ButtonBase Style="{StaticResource Button1Style}" Tag="1" />
<base:ButtonBase MainImage="pack://application:,,,/pathToImage.png" Tag="2" />
</Canvas>
</Window>
Both buttons show at runtime, but only button #2 can be seen in the design space. The value line in the setter for Button1Style is underlined and says "specified method is not supported".
I've tried image asset as Content and Resource and it doesn't seem to make any difference. How can I get this image to show in design time and run time? There will eventually be many of these buttons and I need to lay them out specifically on the canvas at design time.
Any help would be appreciated.