What is the data type of background images in a Pages template?

I am trying to teach myself graphic design. The templates in Pages are really nice, but I want to be able to make my own. I was stripping apart a brochure template ("Elegant Brochure"), and found that most of everything on the page was a text box or an image. Once I removed all of those elements there was still a pattern in the background. When I clicked on it, I noticed "x's" on the corners that were immobile instead of the usual white boxes for manipulating images or text boxes. I clicked on the inspector and noticed that these were actually jpegs, but I don't know how to remove them, or in the case of wanting to make my own brochure layout, insert background pictures like that that will stay immobile.

After flipping through "iWork:The Missing Manual" and digging a little deeper, I discovered that those watermark style background images were immobile because they were "Locked." So I clicked "Unlock" in the Arrange menu, and then I could delete or manipulate those images.