You don't actually seem to be using the logo anywhere, though. On your site you write that "I had (have) plans to write an Instant Messenger client with proper support for privacy. This was the logo concept for the project"[1], but this project does not actually seem to exist (at least not in any public manner).
As such, why not just leave the logo of Tox in peace? Especially since it's a FOSS project with noble goals (even if they have yet to reach those goals, which is not surprising considering how early in the development the entire project is) and not some commercial entity intending to profit off of it.