How to set the Appli.Path in XAML code

You just need to use the image as the content for the button, something like this

<Button Grid.Row="2">
            <Button.Content>
                <Image Source="your image file path"/>
            </Button.Content>
        </Button>

[Edit]

With regard to your comment, the only way I know how to do that is to include the image in your application(right click your project, then select add exiting file, and select your image), and then set the image file's Build Action to content. Then you can use a relative address for the image like this

<Button Grid.Row="2">
            <Button.Content>
                <Image Source="image.jpg"/>
            </Button.Content>
        </Button>

[/Edit]

Hope this helps