Apple state what image sizes are accepatable using iBA. They say that any image over 2megapixels will be rejected. In truth I have had some leeway on that.
I imagine that however you make your book, the content needs to comply with the standards set for iBA. IF you intend to submit to the iBookstore.
Images sizes are best taken to be suitable for Retina display and ignore any other options.
2048 x 1536 Res 264, or.... 2048 x 1496 Res 264
This is landscape, swap around to suit Portrait. Just remember to set the resolution first if you are re-sizing.
Once you have your images fornatted to those sizes, you can fix the size on the page as you want it.
Edit: Any critics should know this is how I have added images to ten photo books which have gone into the store in the past 3 weeks.... without tickets !