If the image is always on the same place you should consider to make a master with this image and apply it to those pages.
Another method would be to place the image once and then use the Content Collector Tool. select the image with that tool, in the Conveyor Ribbon which will appear activate link and also Place Multiple Times and keep in Conveyor.
Depends how you will publish the book. The second method is preferable if you need to publish it electronically like PDF/UA or EPUB, the first method would be good, if you want only to print it.
File sizes increase as you edit because ID is storing the change data during the editing session so you can use Undo. This data remains in the file after close, too, unfortunately, and becomes dead weight. Doing a Save AS will remove any undo data, as will running through IDML.
Wow, that was exactly what I needed, Thanks so much!