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.

In general, save color/b&w pictures at 300 dpi examples: photos, drawings... and, line art (black and white only--no gray) at 1200 dpi examples: text only logos, outline shapes...

If you are using pictures from your digital camera they will work just fine if they are jpgs; the quality of jpg images from digital cameras seems to be much better than jpgs that are used on the web. You must do the math to make sure that it is high enough in pixel resolution though. For instance, if your camera puts out a typical image of 1280 x 960 pixels at 72dpi you get about 17" x 13" of photograph (at 72dpi); this is the same amount of detail as an image which is 4" x 3" at 300dpi so it's safe to reduce or enlarge that image in Quark up to about 4" x 3" in dimension.