You can't.
The browser will set up several simultaneous connections to the server,
the first connection will download the page, the other connections will
download the external elements (CSS files, javascript files, images
etc.) in the page in the order they appear in the HTML. When one
element is downloaded then that connection is used to download the next
element that has not been started - remember several are being
downloaded simultaneously. There is no control on how much bandwidth is
allocated to any particular item, or how long it will take to download
it, hence images will appear to more or less download at random.
