How did you create the png files? There must be something different between the smaller images and the larger. Are they from different sources or created by different apps?
To see how it will print Control-click on the page and select Preview Book in the contextual menu:
Create a pdf file of the book and check for errors before ordering. Save the pdf file to compare with the printed version when you receive it.
Thanks. In the PDF all the images have no backgrounds, just as I designed them. The only conclution I can draw is that for some reason for pictures under a certain size, iPhoto previews them on a black background. Same thing with thumbnails. Who knows... anyway, problem solved so thank you very much for your help.