The following link points to an article of Scott Mitchell on using inline Images and CSS sprites to improve the download time of page.
The number of HTTP requests a browser makes to download a page can be reduced by usingf either of the two techniques: Inline images and CSS. But the primary challenge in using these techniques lies in creating the sprites and CSS rules.
The ASP.NET Sprite and Image Optimization library is a free, open-source library from Microsoft that simplies working with inline images and CSS sprites by automatically creating the sprites and CSS rules. This above article showed how to get started with the Sprite and Image Optimization library in WebForms and MVC applications.