For best print quality, make sure that the photos or illustrations that you have scanned are at least 300 dpi or slightly greater at the size they will be in your document.
Scans are composed of a specific number of pixels. When you increase the size of an image, the size of the pixels increases also. If the original document is a low resolution (a 72 dpi image copied from the Internet, for example), then the resulting image at a larger size will be jagged. The image could be resampled to 300 dpi to compensate, but then the image will be blurry.