Rotate or flip image through Y-axis

I've been trying to work out how to rotate an image through the Y-axis and display its mirror image on a web page. One of the problems that I faced was trying to find the right word(s) to search on as rotate (so I discovered) is used to refer to rotation through the Z-axis most of the time.
I finally found a solution in Silverlight 3. Here's the code...
<Image Source="Jellyfish.jpg">
    <Image.Projection>
        <PlaneProjection RotationY="180" />
    </Image.Projection>
</Image>
...which does exactly what I'm looking for.
My favorite image editing program (Paint.Net) refers to this as Flip Horizontal which I think is completely misnamed.

<PlaneProjection RotationY="0" />

  <PlaneProjection RotationY="180" />