Hello, you may use our flipbook software such as Boxoft PDF to Flipbook to change the inner and outer background image.
Here are simple steps in detail:
Step1: Open Boxoft PDF to Flipbook application, start a new flipbook project;
Step2: Choice Float template from template box, import PDF from Local PC for conversion;
Step3: Find out "Background" bar in "Design Setting" panel (Design Setting -> Flash Display Settings -> Background Config -> Background)
Step4: import photo in "Out Background File" and “Inner Background File”, you can also define the background position in flipbook.
Step5: Click "Apply change" to refresh design settings, then you will see the background both inner and outer has been changed.