With the above caveats about "what the hell is a user anyways", why not approach it from a slightly different direction: tail the log file. Write a small daemon that constantly tails the apache log file (preferably just the one for the virtual host if there is one) and updates modifies a counter everytime it sees a new line in the file. You'll probably have to filter out multiple image requests on the same page, but it should give you an idea and be easier to setup.