Images in XAML ResourceDictionary disappear on ToolBar when Menu opens

I have started to move various common Images into a ResourceDictionary and noticed an odd behavior in my WPF application. If the Image is used in a MenuItem and in a Button on a ToolBar, when I open the Menu the image disappears on the Button.

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Image x:Key="NewImage"
           Source="/SomeApplication;component/Resources/NewDocumentHS.png"
           Stretch="None"/>
    <!-- ... -->

Relevant XAML from the Window:

<Menu>
    <MenuItem Header="_File">
        <MenuItem Header="_New"
                  Command="{Binding NewCommand}"
                  Icon="{DynamicResource NewImage}" />
<!-- ... -->
<ToolBarTray>
    <ToolBar>
        <Button Command="{Binding NewCommand}"
                Content="{DynamicResource NewImage}" />

I assume this is a caveat of resources in a ResourceDictionary, but I am unable to discover the appropriate fix for this. Behavior occurs with both StaticResource and DynamicResource. It also doesn't appear to be affected by if the ResourceDictionary stands on its own or if it is merged with others. No other resource shares that key either.

Edit: Additionally, adding PresentationOptions:Freeze="True" to the images did not change the situation.