Three images, actually (normal, hover and active). Six if you have buttons in two different colors. Twelve if you need the buttons in two different sizes.

You need a truckload of CSS to define how a button looks but you only need to do it once. Creating arbitrarily colored and sized buttons is as easy as adding one or two lines of CSS. And at some point in the future we will actually be able to drop the prefixes and the CSS will become quite elegant and compact.