dialog menu backround and freeze

is there any way to make it so when a player opens the dialog menu, a backround picture will come up behind it? like one of the wallpapers form blizzard

and also, i have a button called description, i made it so when a player click it, it will have a auto timed message come and tell you stuff, how do i make it stay in dialog mode? with the backround behind it, then when they are done reading it, i will make a trigger to exit the backround when they type "exit"

A filter mask can make a background, but you will have to import the picture, and after exiting dialog mode, simply bring it up again instantly, and of course ending the filter mask and closing the dialog is a pretty simple step from there.