Pictures are embedded in phpBB by generating an <img> HTML-tag. This is interpreted by the browser. The browser sees the tag, reads the URL in it, and downloads the pciture from whatever location is entered. phpBB has nothing to do with that process, al it does is generate the appropriate HTML.
How the browser handles it, is up to the browser; some browsers already start laying out the page when they have loaded all the HTML but not all graphics yet, others wait until everything is available and then lay out the page (well, actually, most browsers use the former method, but hey..
) How fast the browser is able to load the picture depends on lots of factors, such as the speed of the connection and the speed of the server serving the picture.