Portfolio

To make the page active, click the Design tab -> Page Fill.

You can fill the entire page background with a colour, gradient or picture.

Fill

NB: When you click Page fill it becomes the active element, all styles you apply will apply to the page and can affect other elements on the page. You can only fill the given options (colour, gradient & picture).

Make sure you click another element and make it the current active one before you apply any other styles

To make a picture fill the whole page,  go to Format tab ->Background->Cover. You can adjust its position; left, top, center etc or make it a pattern if it does not fill the whole page (contained)