Gonzalo Ayuso had previously shown how to dynamically add watermarks to your images with the help of a mod_rewrite rule. One of the side effects of this is that, with a high load on your server, things can get bogged down quite quickly. His alternative? Add the watermarks to your images with Gearman workers.
In this second solution I will use a gearman worker to generate the watermarks. The benefits of gearman is the possibility of use a pool of workers. We can add/remove workers if our application scales. Those workers can be placed even at different hosts, and we can swap easily from one configuration to another.
He includes some sample scripts to illustrate the process - a Gearman client that'll call the watermarking process and a worker that takes in the image and transforms it with GD to add a new layer for the watermark.