Not much! Maybe use it but they have to credit you? This is a bit tricky. How would you even keep track of this and how would you be able to track them down to see if they violate your copyrights? You would have to write up your own agreement and the client would have to sign it but you would have to run it by customer support to make sure you would have permission since the exchange of personal information isn’t permitted. You won’t know. To be honest, I am not sure if Fiverr is the right place if you are that concerned about the usage, commercial license etc.