hello, i have been trying to do the same for my page, although i am an inexperienced user ...
I went to public_html/themes/classic/assets/img
and copied the background image there
then went to
public_html/themes/classic/assets/css
edited the custom.css file and i added the code there
#wrapper { background-image: url(../img/bkg.jpg); }
with my background images name instead of bkg.jpg
Any help?