Thanks for the tip, Drago! Does that mean I'll replace those icons in google drive since that's where I'm saving my project? Or do I have to save it in offline mode?
Wow, I was expecting something like in Cocoon.io where you just upload the image of your icon, then it's done. xD
It's in C3 Itself, in the Project bar on the right.
Just double click and change the images to your image, with the correct size.