How to change the background of a Photomerge project in Photoshop Elements 10?

In making a photo collage in Photoshop Elements 10 I find that using Photomerge is the simplest (the least steps toward an end product) method but it has one problem which I cannot find a solution to- how to change the background color. I am following these steps: File-New-Photomerge Panorama-Interactive Layout, then I browse for my pictures and select them. Next I press OK and I am at the main screen with the ability to arrange my photos and adjust the individual layers. The problem is that when I save the end result the background is white. I can find no way to change this as it is not listed as a separate layer and I can find nothing addressing this online. All I want to do is change the background of my picture collage (the white and grey checkerboard part behind the photos I am manipulating) to black without jumping through a series of unnecessary hoops. Is this possible, and if so, how? Not to be offensive but if you don't know what I am talking about I would rather you just ignore this question. Thanks.