It looks like you've got the GD library figured out. It should be pretty easy to take this to the next step.
Your first option is to just generate images for every user once a day (or however often you want) and store them on your server, then give users images like:
[enter user id here].gif
The better way would be to use a PHP page that generates a 100% up to date image every time, like this:
[enter user id here]
In order to do that, you create the PHP page with all of your image generation code. Look at $_GET['user_id'] to figure out which user to generate the image for. Then use headers in PHP and a web browser will treat the PHP page just like an image.
Keep in mind that a lot of forums don't allow you to use images with a .php file extension. There are other ways around that, but it gets somewhat tricky.
See if that gets you started. You may have some questions about how to do one of the above options, so please ask if you do. I just wanted to give you the high level overview first.