[flexcoders] Changing background image in datagrid - does not clear out

[luvfotography]

Hi, I've got a datagrid in AIR, alpha=".5" backgroundSize="100%"

and I'm dynamically setting it with:
var picurl:String = 'http://mysite.com/mypic.jpg';

setStyle("backgroundImage", picurl);

and this works, however after 2 or 3 background changes, the pictures don't 
clear and they keep layering each other with some transparency.

I tried clearStyle, no luck,
any way to clear out the old image?
thanks