Using image sprites is sort of a last resort after you have done as much as you can with other optimization techniques. A sprite is an image that has all of your other images inside of it, so if your sprite has 10 images, you'd be getting rid of 9 HTTP requests and speeding your site's loading time up by the time for each of those 9 requests' latency.
"To add CSS or JS that should be present on all pages, modules should not implement this hook, but declare these files in their .info file."
CSS files can be added to a .info file using the following format: