It appears that exporting a project to XAML so it can be included as a resource file generates markup that is invalid. When I add the generated file, I get a bunch of errors about names already being defined.
Here is a snippet of the generated XAML:
<Viewbox x:Key="Media-Pause">
<Grid Width="48" Height="48" Visibility="Visible">
<Grid Name="backgroundGrid" Visibility="Visible">
<Rectangle Fill="#FF000000" Name="Rect" Visibility="Visible" />
<Ellipse Fill="#FF000000" Name="Fill" Visibility="Collapsed" />
</Grid>
</Grid>
</Viewbox>
<Viewbox x:Key="Synchronize">
<Grid Width="48" Height="48" Visibility="Visible">
<Grid Name="backgroundGrid" Visibility="Visible">
<Rectangle Fill="#FF000000" Name="Rect" Visibility="Visible" />
<Ellipse Fill="#FF000000" Name="Fill" Visibility="Collapsed" />
</Grid>
</Grid>
</Viewbox>
The names in bold are what the compiler is complaining about. Until this is fixed, I can't really any of the images other than to include them as actual PNG files, which I would rather not do.