Tattoos and Mehndi

Q: Are tattoos Haram? The two most common reasons I heard is ghusal will be invalid because it covers the skin and because it is permanently changing the body. However, with modern advancements we can now remove tattoos and the tattoo is under the skin so it does not affect ghusal. If both of these are now not an issue, are tattoos now not haram?

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?