Flex :: Photo Gallery - Resizing Image Whilst Keeping It Central

I am using a mx:Resize effect to change the width of an image in Flex (leaving the height to look after itself) but I want the image to remain vertically centered on the page. Whilst I could calculate the aspect ratio of the photo, and work out the new y value to keep it centred, I would have to include a mx: Move effect too, and I am hopful that Flex has a simpler way. This is for a photo gallery where images are loaded at run-time, and swapped about.

So I really would like to avoid recalculating the aspect ratio and vertical position every time the updateComplete event fires, I tried putting my image inside a mx: VBox with verticalAlign="middle", thinking that the box would do all the centering for me, but this only seems to happen when the app first starts - i need it to gradually adjust the position during the period where the effect plays (to keep it central even when part-way through).

Here's a stripped-down example (which doesn't work):
<mx:Application xmlns:mx="[URL]" layout="absolute" width="600" height="600">
<mx:Resize id="fxResize" target="{imgPhotoA}" widthTo="350" duration="500" />
<mx:Button x="10" y="10" label="Resize" click="fxResize.play()" />
<mx:VBox id="boxPhotoA" x="100" y="20" height="500" width="350" verticalAlign="middle" borderStyle="solid" verticalScrollPolicy="off" horizontalScrollPolicy="off">
<mx:Image id="imgPhotoA" width="60" maintainAspectRatio="true" source="[URL]"/>
</mx:VBox></mx:Application>