Tattoos and Mehndi

Q: I have heard lately from a lot of my male friends of men and women who are tatooing their bodies with mendhi. Is this permissible?

Q:

1. A brother accepted Islam, but previously was a Christian who has tattoos on his arms, some are facial images, possibly Christian crosses too, is it permissible that he tattoos those images with plain ink so images are not visible to see what it represents or should he leave it alone? 
2. Will his wudhu and salaah be valid with the tattoos?

Q: Is it permissible in Islam to have a tattoo on one's body? I performed salaah behind a person who has a tattoo on his skin. Is the salaah valid?

Q: If a person has a tattoo which is into his skin,will his ghusl be valid?