It's found on this page, I'm assuming it should be the same on your wiki. On ours, the part that says background: #ffadc0 url( comes right before the URL address of the image that you want for the background. I created a page with a single URL that shuffles between multiple images for this wiki, I can help you with that if you want yours to do that too.
I had to create a website at zxq.net and use a program called FileZilla to shuffle them. If you just want one image, you don't have to do anything like that, but if you want them to shuffle it's pretty complex.
Please check the Nicki Minaj Wiki:Community Policy page. Read and edit it if you think that the page can be improved. You can also include more inappropriate or banning reasons (I trust you'll make good ones).
If you want to discuss the page more deeply, do it in the here in the forums.