A request for a web page is received at a proxy server. The request originates from a client network application of a client device. The requested web page includes multiple references to multiple images. The proxy server retrieves the requested web page. The proxy server modifies code of the retrieved web page such that the client network application will not, for each one of those images, initially request those images when parsing the page. The proxy server also adds code to the retrieved web page that, when executed by the client network application, causes at least two of the images to be requested with a single request. The proxy server transmits the modified web page to the client device.