The commands are defining the image's height and width in pixels.
Pixels are little colored dots.
If you put enough of them close together, they form a picture. Those with vivid imaginations know that the height and width commands can be used to change an images shape. If the image is really 50x50 and you put in height and width commands of 150 and 10, you will distort the picture pretty well. For an example, see the Manipulating Images lesson. A better way to use the commands is to define the exact height and width of an image.