With the price of SSDs, I can appreciate your concern, but as I said, the wallpaper itself would not be rewriting at each change. But it is possible that some other write function could occur that is in some fashion involved in the process, I really don't know, not being a MS programmer or a hardware developer. In any case, there are other processes that do tend to write on a continuous basis. This can be seen by installing a System Tray program called DriveGleam, which has a both a red and green LED to indicate read and writes as they occur.
If you wanted to know what is going on, you could install Filemon to monitor file activity and Regmon to monitor registry activity. However, if I could afford a SSD, I might feel that my time was more valuable than what any possible wear might cost. Obviously, neither reads or writes can be avoided. Still, I like DriveGleam, because it can alert one to worse activity.