Please refer to this link: http://askanalim.org/node/2664
Also, the tattoo must be under the skin in order for it to be not one that prevents water from reaching the skin for purification i.e. ablution and ghusl.
Furthermore, it must be emphasised that tattoos are not something this is appropriate and narrations tell us that they are not approved of. So while jurisprudentially it is allowed, in terms of a higher level of spirituality, it should be avoided.