add background to jframe in java with netbeans

hey all i want to add a background on my frame i searched the INTERNET but all i found is a code !! but net beans is generating the frame code automatically !!

i don't know where to put the code in there !! and how it works

what i want is a dynamic(stretch) background to my frame !! how it can be done ? and if there is a code where to put it and how to implement it !! thanks

to make it clear i want to add a background than can stretch automatically to my jframe but my jframe code is generated automatically by netbeans !!