J.P.
It's just a tatoo. An expression of herself. Let it go. It could be a lot worse. And she is the one who has to live with it for the rest of her life. At some point you are going to have to allow her the freedom to express herself as she chooses and as long as she's not hurting anyone else, what is the big deal? She is 20, which means she is a young woman, but I dont think charging her rent is going to make her tatoo go away. It is just going to drive a bigger wedge between the family and cause her to think she can do "whatever" she want because she pays it. Especially if you dont "need" the rent.
Sit down with your husband and help him let it go. Then talk to your daughter and maybe ask her why she got it and what it represents to her. You might even learn a litte about her! Good Luck!