This may be impossible to accomplish.
If you have an image and below that you want sizeable text to appear underneath the proper picture, I would give up on that.
I formatted my first book like that. Since the text can be resized by the reader, there's a good chance the text will spill onto it's own page, or above the next pages image.
I'm assuming you are separating each image and text by chapter, so they will get their own page. But once the book is opened by the reader, there is no telling what font size they have chosen. Since the text no longer fits below its proper image, it moves to the next page to keep the text together.
If that makes sense.