I toyed around with pixel tracking like this before with PHP and the GD library. You just create an 1x1 white pixel and set the file extension to .png or whatev

I toyed around with pixel tracking like this before with PHP and the GD library. You just create an 1x1 white pixel and set the file extension to .png or whatever, and as long as you've configured the server right, it will execute and return a pixel. But then you can do all the other tracking you want. And the user doesn't know any different. That said I won't use that in the future but it's scary how easy it is.