How to tween scale with keeping the bottom center position ?

Little bit more explanations : using a marker like this one, the bottom pointing on a location.




I make this action : onover="tween(scale,1.5);"
Works well, but scale origin is top center, so the bottom of the marker moves.
I would prefer scale origin to be bottom center, so the marker keeps pointing to the same place.

Sure it is very easy, but I can't find how


Hope you can help !