Some websites set a background image to advertise, e.g.
<body style="background-image:url('images/adv.png')">
I'm wondering how I can write an AdBlock Plus filter in order to hide the <body>
background.
If you are not strictly sticking with ABP, you can use Stylish to do it in CSS way. In fact, it is my preferred way to do things like that.
body { background-image: none !important; }
Of course you can just make the ABP filter containing whole URL, this will affect not only background, but I can't think of the reason why not to block the URL entirely.