WpForo custom topic icons?
I'm really sorry but there is no way to upload your own topic icons.
However, we suggest you check out the following support topic, here the wpforo members provide some CSS solutions: https://wpforo.com/community/how-to-and-troubleshooting-2/replace-font-awesome-icons-with-images/
For farther support please register and open a new support topic in wpforo support Forum. This is the gVectors support forum. Here we only support the wpforo paid add-ons.