“Silk” is a smooth, free icon set, containing over thousand simple icons that are free to use for any purpose. All those at famfamfam.com ask is that you include a link back to there web page in your credits famfamfam.com/lab/icons/silk/.
The images here are loaded as background images through CSS. What is more is that the images are referenced as Data URI's. The data URI scheme is a URI scheme that provides a way to include data in-line in web pages as if they were external resources.
The data is represented in Base64 encoding. It's worth noting that Base64-encoded data is about 4/3 of the original data size, or about a third larger than equivalent binary images. There are also not cached.
With that noted, there are advantages to using Data URIs. Data URIs reduce the number of HTTP requests. Negotiating a new HTTP request is often the bottleneck on a broadband connection. With small files (like these) the overhead of establishing a HTTP request can actually represent more transferred data than the image itself. This is the reason techniques such as CSS image sprites have been developed. Basically, if you can avoid requesting an extra file, not only does it save your server the work of looking up the file, but it also saves your user the download time.
Web servers can include the image in the content they deflate/gzip before they send it to the browser.
For your connivence, here are is the famfamfam.com Silk icon set as data URIs. Copy and paste as needed.