In order to place a single photo across two pages, the two pages must be linked as a two-page spread. A linked two-page spread does not have a yellow border in the gutter area between the two pages (between the red trim lines).
To link two pages together, click on the Link button in the second-level toolbar.
Once the two pages are linked, you may place a photo (or any other item) onto the spread and move or size it so that it crosses across the gutter. Please note that the page will be trimmed where the red line appears and the portion of the photo that covers the red line may be hard to see because of the binding process. For this reason, placing critical items such as type, where you have to see every letter in order to read it properly, is highly discouraged.