Among the 'homosexuality is evil' threads, the 'tattoos are sinful' threads and the 'Islam is unduly mysogynistic' threads there is a common set of criterion a person should address.
1. If a person is to assert an instruction in a particular book of the bible is relevant and should be upheld and followed, then they should uphold all intructions within said book, on matter of principle; for if one instruction within a book of instructions is not valid, the whole book is therefore not valid.
2. Which book does said instruction come from?
3. Do you, as a believer of that particular instruction, also follow all other instructions in that book? If not, then you should re-examine yourself in that light.
So, let's answer hose questions for tattoos.
Now, the instruction against tattoos comes from Leviticus (19:28 to be precise) and commands for people 'do not mark your bodies with tattoos'. If this instruction is valid, then all other of Leviticus' instructions must be valid, otherwise which do you uphold and which do you not?
Other instructions within Leviticus include the instruction that a woman must be killed if she wears fabric of two different materials, that men who sleep with men must be stoned to death, that daughters can be sold into slavery, that any person who touches the flesh of a deceased pig is unclean, and that fortunetellers should be put to death, among many others.
Do you consider those instructions as commands to be upheld? If not, then you have no reason to consider tattoos sinful,and you might as well tear Levitius out of your bible.