Consider carefully before you do.
First find a tattooist who is reputable for and specialises in portraits. It's not an easy job to get a perfect likeness tattooing, unless the portrait is a fair size, and the less intricate the better.
Also check with the tattooist on where you are thinking of getting it. Arms and legs are a very uneven canvass, so the design you choose should also fit the area where you are getting it done. Ideally, portraits work best where the area is even/ flat as possible, as curves on biceps etc, can distort the finished product.
I have an Elvis tattoo and it is not very good