Try /help /background .... since im not sure exactly what youre trying to add a background to... All the options for /background are listed there. With more info i can help.
This is correct, presuming the status window is the window behing the dialog. If you opened the dialog with a channel window active then the channel would get the background.
You should stretch an icon control across the whole dialog and put a picture in that if you want your dialog to have a background.