It’s useful to know the dimensions of an image (in pixels). For a given picture, more pixels generally means higher quality, higher resolution, and an ability to be printed larger without looking all pixelated. Almost every graphics program has a way of showing you an image’s dimensions but you can find that info without even opening the image up. Just click on its icon once, then go to File/Get Info (or ⌘-I). You’ll see the dimensions in the More Info section.