simple PHP app: sprites and adding nav icons

The last part of the extra credit in the header and footer section asks us to add a new icon. while I generally know how to add an icon as a CSS background the other icons seem to be included as a sprite sheet?

This is the first time I've come across sprites. So I don't really know how to use them, nor do I have an idea how to modify/add to the respective png file. How do I know where to place the icon, what size it should be etc? Is there another advantage to using a sprite other than load time? And if I added an icon just as a new background is it possible that I am inadvertently layering multiple backgrounds, because the sprite file extends into the space for the new icon?

Does anyone know whether sprite sheets have been part of another Treehouse tutorial? If not, does anyone know a good place to start, if I wanted to know more?

Thanks!