In the top right hand corner of the Book Module under <Book > change from <PDF> to Blurb.
With Blurb selected you will get lo-res warnings when the resolution is too low. Later on you can change back to <PDF>.
See screen shot:
You would not get a lo-res warning if you used original images from your camera. But if you either use small JPGs, or you crop an image severely and then use it for a full page image, the image will be too small to be used.