The aspect ratio of your camera probably isn't the same as your monitor's. So you can either do as above, or crop it such that the aspect ratio becomes the same.
In most cases, a picture loaded right off your DSLR would be much larger than a LCD monitor's native resolution. So it is "compressed" to fit on the screen rather than stretched. And as mentioned by the bros in the above post, the aspect ratio of the picture does not match that of your screen, so you'll probably be required to crop according to the aspect ratio and resize to the native resolution of your monitor. This should make the picture look sharper.