Believing you are talking about CSS Sprites, it's a good practice of handling images(especially for navigation) and also the number of HTTP requests.
Get the image once, shift it around and only display parts of it, saves the overhead of having to fetch multiple images
You don't use a whole bunch of small images (such as icons and menu buttons) but collate them all into one larger image (which requires only one HTTP request to load it instead of a whole series of them) and then use CSS to select and display only a certain rectangle from that image (using width, height and position offset) as a background image.
There are n number of tutorials and even CSS Sprite Generators(but I dont have any personal experience on using generators) that would help you explore the technique.
FYI: The best practice of working on sprites is either before designing the website(where you will assume the required images) or at the end of the design(by then you will be sure of the required images--more recommended--)
I guess this would be a best place to start with:
Google them! Explore!!