fluid content shows as boxed in sections?

Hi Tom,
I have set up my sections to be fluid. But when I choose a colour background other than white, the section appears as boxed... My container settings are 1 container for all pages and no side bar. I watched your little video tutorial on sections and it looks like that you always pick up a background image when the colour is not white. Am I right? If this is the case, what should be the size of the background image that needs to be uploaded to cover the edges?
Thank you!
Anne