Figured it out. Had to do the binding to RelativeSource.Self.. Add two triggers for true and false. Ignore the converter.. I needed one for the image I was using.
<style x:key="MyWindowKey" targettype="{x:Type test:MyWindow}" xmlns:x="#unknown">
<setter property="Tag" value="{DynamicResource {x:Static SystemParameters.HighContrastKey}}" />
<style.triggers>
<datatrigger binding="{Binding RelativeSource= {x:Static RelativeSource.Self}, Path=Tag}" value="True">
<setter property="Background" value="{DynamicResource MyWindowBrush}" />
</datatrigger>
<datatrigger binding="{Binding RelativeSource= {x:Static RelativeSource.Self}, Path=Tag}" value="False">
<setter property="Background">
<setter.value>
<imagebrush imagesource="{Binding Source={x:Static test:Images.MyImage},Converter={StaticResource ResourceKey=ImageConverter}}" stretch="UniformToFill">
</imagebrush>
</setter.value>
</setter>
</datatrigger>
</style.triggers>
</style>