That would be my guess too. Publishers don't really understand eBooks or how to correctly design and create them. If an iBook required some special code, then few publishers would understand it or get it correct. Making the images work auotomatically just makes sense.

EDIT: I think image size might not be so important either, though I'm just guessing. The reason I suspect this is that I've read a few history books with maps and charts and various other images, and when I expand them on my iPt4 or the iPad the images are extremely pixelated. They are so pixelated that text within the images is impossible read. In other books the expanded images are much better. So I suspect a small resolution image will expand just like a large resolution image, but the small resolution images will look terrible.