Help placing Header Picture with Cellfuse Skin

Hello all

I am working on my forum and looking to place a transparent picture in background of the main header cell (> http://www.ivisitfr.com/agora)
I have open cellfuse_cs.css but can't find a way to place my argument.
Is it something like "background:url(http://www.ivisitfr.com/agora/img/logo.jpg) no-repeat center"  ?

Regards