Story apps: subtitling of images in portrait mode

We have a number of printed story books with portrait images that we may turn into story apps. For this kind of images it would be nice if the display options could be changed:
If the phone is held in portrait mode, have the image display full screen and text shown as subtitles (when audio/text synchronization is enabled). When the phone is turned to landscape orientation, the picture could be displayed smaller on the left of the screen, having the text scroll next to it. So basically, the display option is reversed compared to what it is in SAB/RAB now.
At the moment, we are struggling to do this kind of app. Either we have to use a standard SFM file (e.g. XXA) and add images to the text that can then be scrolled with the text. Or we use the Picture Story Book standard and the image will be displayed rather small at the top of the page. We can’t really crop the images to landscape.