I'm not 100% sure I understand your question, but . . . .
Are you trying to create a picture in layers, with a basic backdrop as the the background image, and then you will layer something on top of it?
To create a basic backdrop you might try to "render" the background by using the clouds filter or some other filter to your liking. You will have to experiment both with the filters, as well as the foreground and background colors (found on the tool bar).