Need help changing a custom window skin

Is there some script or way to change the window skin? But I mean, not everything, just the one with conversations. 

I want to use this window;

But I can't find the way to do it, so... any suggestions, please?

Thanks.

I know some folks do it with setting pictures via events. I haven't played around with that method so maybe someone else who has can say how they do it ;)

but here is a script that sounds like it'll do it

http://galvs-scripts.com/2012/12/02/message-background/

 Thank you so much, it worked perfectly!!