I've done some experiments, and I agree with jgoguen. It sounds like it is how the book was created.
Attached is a sample book with pictures and chapters in various layouts:
- First chapter has full screen picture in the middle of the chapter
- Then another chapter followed by picture in a separate file and another set of text in separate file.
- Then a chapter that starts with the picture and has text afterwards.
In all cases, the picture is wrapped in SVG to display it full screen. And they work as I expected. The first has page numbering across the full chapter including the picture. The second restarts the page numbering for each file. The picture and later text file don't have a chapter name displayed. The last one starts the page numbering with the picture and keeps going from there. It doesn't display chapter name.
My guess would be snowstone's book does the second one. But, it might be the last one. Splitting the picture into a separate file should make it easier to guarantee it is displayed on a separate page by itself. But, it does mess up the page numbering used for kepubs.