R – How to display an image within the WPF Aero Glass Titlebar

System.Reflection.Assembly.GetExecutingAssembly().Location¹

Combine that with System.IO.Path.GetDirectoryName if all you want is the directory.

¹As per Mr.Mindor's comment:
System.Reflection.Assembly.GetExecutingAssembly().Location returns where the executing assembly is currently located, which may or may not be where the assembly is located when not executing. In the case of shadow copying assemblies, you will get a path in a temp directory. System.Reflection.Assembly.GetExecutingAssembly().CodeBase will return the 'permanent' path of the assembly.

I figured out how to get it to work. I set the entire window to have the aero glass effect on it using the native API's and then a create a LinearGradientBrush for my background of the window. In the brush I used the Alpha properties of the brush and set the stops to have the top of the window go from white/opaque to white/transparent all very close to the top of the window.

<Grid>
        <Grid.Background>
            <LinearGradientBrush StartPoint="1,0">
                <!-- This gradient stop is Fully transparent. -->
                <GradientStop Color="#00FFFFFF" Offset="0.0" />
                <!-- This gradient stop is fully opaque. -->
                <GradientStop Color="#FFFFFFFF" Offset="0.1" />
            </LinearGradientBrush>
        </Grid.Background>
</Grid>

I 1 up'd Mikko Rantanen's answer because I used the article to add the glass effect I didn't have the code handy and it was a good simple article.