MyBB Community Forums

I want to resize my forums & add background image like this!!

Newbie here...please can anyone help!!

for the width see replies here
for the background, you can use css background for body element

editing theme style sheets should be done through admin panel (in advanced edit mode)
admin panel >> Templates & Style >> themes >> click on the active theme >> global.css >> Edit Stylesheet: Advanced Mode (tab)

hard refreshing browser (eg. press CTRL + F5 on forum index) might be required after saving the changes to style sheets.

Resize problem is solved...but how to change background??

Change the background from #content in global.css