You should supply your images for print using a resolution of 300 pixels per inch (ppi) 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 300ppi and then enlarge the picture by 200% in your layout program! This is another reason why you should NOT use images that are copied from websites; they are typically 72ppi and will look very blurry when printed on a press.
If you are using pictures from your digital camera they will work if they are high resolution jpegs. You must do the math to make sure that it is high enough in pixel resolution. For instance, if your camera puts out a typical image of 1280 x 960 pixels at 72ppi you get an image size of approximately 17” x 13”. This is equivalent to an image that is 4” x 3” at 300ppi.Do not enlarge your images so their print resolution is less than 300ppi.
(Actual Image Resolution) / (scale) = Effective Resolution
If you open the image in Adobe PhotoShop or another image editing software, and it looks like junk, it will print that way.