The human eye cannot detect the squares of color if there are 300 or more in an inch. For clear and crisp printing, images at their final size in the layout should be at 300dpi (dots per inch = squares of color in an inch) or 400dpi if the images include text.
You should scan your images using a resolution of 300dpi at the final dimensions you intend to use them so that your colors will look smooth, and hard objects will look sharp. In other words don't scan at 300dpi and then enlarge the picture by 200% in your layout program! This is another reason why you should not use images that are lifted from websites; they are probably only 72dpi in resolution and will look very blurry if printed on a printing press.