Well, here was my thought... I'll write the code tomorrow.
Have the 'generator' take their start date, cost per cigarette, cost per equivalent in ecigarettes, and startup price and generate a link such as:
domain.tld/signature/MTI1NDYyMTYwMC0wLjI3NS0wLjAwMjUtNjU=.png
Now, take mod_rewrite and rewrite that url to:
domain.tld/signature/generate.php?v=MTI1NDYyMTYwMC0wLjI3NS0wLjAwMjUtNjU=
Once you have that, if you can't tell, it's a base64_encoded string.. decoding it should give you the string 1254621600-0.275-0.0025-65
Now, match against it to set your variables up, and generate the image.
This would save needing to make a bunch of png files (which are really just php files with a custom file handler) to generate the signatures.
Make sense?
Like I said, I'll write something tomorrow, complete with graphics.