multisite hit counter, includin session statistics

Hi! I want to write some multisite hit counter, but including session statistics. Anyone knows how to do that? I mean: I need to place some link on the page, on not allways php enabled site - probably in form of blank image, which will run script on my server. Ex.: <img src="http://my_serv.org?site=0011">. This way I can produce statistics including: - page access number - IP adresses of users ... but can't(don't know how): - number of unique users - average time users stays on site etc. Any hint? Thanks, Minca