After doing another theme from scratch, and testing 16 more themes, and everything fails, I'm going to re-send this bug to XFCE guys. The notification dialog takes bitmaps from the wrong place. No matter if you specify another source, it ALWAYS gets the gnome / xfce / lxde panel background. It's impossible to use separate images for both desktop elements.
After doing another theme from scratch, and testing 16 more themes, and everything fails, I'm going to re-send this bug to XFCE guys. The notification dialog takes bitmaps from the wrong place. No matter if you specify another source, it ALWAYS gets the gnome / xfce / lxde panel background. It's impossible to use separate images for both desktop elements.