In Zno Designer, make sure you on "Edit Page" and on the correct spread with the image you want to expand.
Next, to expand any frame to exactly half a spread click the image once and a black bar will appear beneath the image. Select the center "Expansion" button.
You can then choose to "Expand to Left page" or "Expand to Right page."
If you have any further questions, please email support@zno.com.