I'd like to keep the signature banners if possible as I use them on some forums.
If you want to autogenerate them, you might consider using PHP with the GD library. You may find some info there :
http://www.libgd.org/Main_Page
http://fr3.php.net/manual/en/ref.image.php
As I'm not knowledgable in Php, I don't know if it's the most appropriate library but I'm sure you can find one that fits your needs.
As a sidenote, having a compact version of the banners would be really sweet since I'm not a fan of huge signatures.
Here is a test of what the signature could look like :
You might be able to reduce it a bit further by using smaller fonts for the sales total and using smaller percentage bars.
But since you currently generate them manually, I guess it's too much to ask for this compact variant for now.
I hope you manage to automate the process so you can keep the banners and make variants at the same time 
