I have been working with a user to design changing signatures with PHP. We created a site to do it on.
you create an account, then press create signature, then enter IMGUR LINKS ONLY for the images. It will then give you a URL to use as the "image url" that then rotates through your amount of signatures. You can make multiple, as we havent' made it to the point where you can edit them yet. I will release the script later.
again, here is the URL for it;