In laymen terms, many routers have settings to prevent various types of attacks from taking place, including something called "SYN floods", which can look like a large number of incomplete TCP or UDP sessions coming from the same host (server in this case). Keep in mind that each image (and other components) in a web page that is being downloaded to your browser is considered a separate session and is incomplete until the download of that image is finished.. Many routers are configured to kill incomplete sessions over a maximum, quite often with a default setting of 10 sessions. If the page has more images and network latency is a little above average, you could start seeing dropped images.