More about sprites
Sprites allow you to conserve bandwidth and improve performance by reducing the number of requests to your web server. They do this by storing all of your buttons and icons in a single image. When you upload an image, the sprites should be generated automatically.
However, the WHM interface contains a Generate Sprites link in case the process encounters a problem. The link is intended as a fail-safe and should be used if the changes you have made to the appearance of the cPanel interface do not take automatically.