Anyone could explain how a browser works to detect if a new version of a graphic file is available on the server? For example, you start your browser and a page is loading pretty fast, assuming most of its content, such as graphic, is cached. But, at startup, as it is the case for my setup, if a newer version of the file is available on the server, the new version of the graphic file will be used. How does that work exactly as far as development goes?