it works like this, that when the page is loaded, the record is inserted into the database as a bot visit by default. Then, when the browser loads the joomlawatch icon, this record changes to a real visitor one. So maybe your friends have browser that doesn't load images. Caching mechanism is avoided here, because everytime there is a img.php?rand=RANDOM_NUMBER so the browser should make a new request everytime.
I don't think the proxy server could cache these icons as well, because of the link that's everytime different.