Jazak could you refine your question a little please? What exactly do you mean by putting an object in the background?
A few shots in the dark:
Are you trying to have an object spawn as part of the scrolling background? If so, take a look at the pin behavior for your spawned objects.
Are you trying to modify the background itself so it looks different? If so, modify the actual image, unless you mean for it to be procedurally generated, in which case again use the pin behavior.