image scripts?

I don't know what to call this.. but there's websites where you have a png image and it does things like count users online.. i was wondering how they do this? is it php?

It could technically be pretty much any programming language. You could do it with PHP, yes.
The easiest way it to simply point an image tag to a PHP page, do your processing, and render the image with PHP's image (http://php.net/image) functions.