<Grid>
    <Image Stretch="Uniform"
           HorizontalAlignment="Stretch"
           VerticalAlignment="Stretch" />
</Grid>

<Grid>
    <Border Width="{Binding ElementName="myImage", Path=RenderedSize.Width}"
            Height="{Binding ElementName="myImage", Path=RenderedSize.Height}">
        <Image x:Name="myImage"
               ... />
    </Border>
</Grid>

<Border HorizontalAlignment="Center" VerticalAlignment="Center">
   <Image x:Name="myImage"/>
</Border>