As I said, it can be done front or back end, you can use the same script I wrote if you want to stick to javascript, or can implement the same in any other language, and then just return the generated image. In fact the algorithm is so simple you can write the raw bytes out as a bitmap without depending on any image libraries if you wanted.

The difference is that it's stateless because it's deterministic. and therefore requires no more than a single small script.