What is the best way to add background ads to a WordPress site?
And which advertisement provider would you recommend for publishers?
You don't generally need to do anything, as such - the ad providers give you the relevant codes to replace the background. That said, you're unlikely to sell any background ads unless you're pretty highly trafficked and specialised, so I wouldn't pin your hopes on them. I use http://buysellads.com for direct ad sales, though I believe the background ads are invite only. If your site performs well on other ad units, they will probably contact you.