"Many artists use only vector art in some of their designs, whether it’s for print or the web. Some clients’ web sites or print material lend themselves to consist of illustrations rather than photos. Other clients may prefer almost all photos to have a more personalized look that has more human interest appeal.
Raster images are composed of pixels, and are often photos. When you take a photo of something, digital cameras save the information in this way. So when you open the image at full size, that’s as big as it should be." Full tutorial