iPhoto '11: resolution
Resolution refers to the number of pixels per inch that an image contains. A photo’s resolution is sometimes referred to as the photo’s size (small, medium, or large).
A digital photo is made up of millions of individual dots of color, called pixels. A photo’s resolution is the number of dots per inch (dpi) that it contains. The higher the resolution, the more dots per inch, and therefore the more detail. Higher-resolution photos look better when they’re enlarged. Some photos look jagged or blurry when printed if their resolution is too low.
You can change a photo’s resolution using iPhoto. Which resolution you choose depends on two things: the quality of the digital camera, and the photo’s intended output (print versus online, for instance).
Camera quality: Photos taken with a camera phone, for example, tend to have lower resolution than those taken with a digital camera. Old family photos scanned and saved on your computer might have lower resolution than a recent photo (film or digital) saved on a CD (depending on the scanner settings).
Media output: If you email a photo to someone or publish a photo on the web, you can use a low resolution so that the photo appears onscreen faster; on a computer screen, it will look just as good as a high-resolution photo. But for prints, higher resolutions give better quality. The bigger the print is, the higher the resolution should be.