Are Tattoos a sin?? Yes and no...

In ancient Egypt tattoos were used for several reasons:
1. Magical significance (animal tattoos...)
2. Decoration (Egyptians were very obsessed with their appearance/vanity)
3. To reflect their status
4. Fertility (females would have tattoos put on their abdomen to increase fertility)
5. To ward off evil (these tattoos were usually found on their wrists and fingers)

I am sure there are more, but these were the most common reasons.

When Lev. 19:28 talks about "no tattoos/body markings", God was trying to take the Egyptian practices out of the Israelites.  He wanted them to put aside the pagan/worldly practices and not rely on them or make them a priority in their lives.  This was to make them rely on Him for all things and to keep their focus on Him not on idols (tattoos were in a sense idols to the Egyptians).  It was also to show the Israelites that it was God who had all the power not the tattoos.

As for Christians today, when we get tattoos we need to search deep within ourselves as to why we want them.  If we are getting them to 'ward off evil', or any other reasons that the Egyptians had them then yes it is a sin because we are taking the focus off of God and making our tattoos idols in the sense that we place more value and power with the the tattoos than God.  Also, if we get a tattoo that is a constant reminder of a past situation/incident and it makes us dwell on what happened in the past, not allowing us to move forward then we are taking the focus off of God and how He can help us through those past situations and use us for His purposes, then yes it is a sin.  Also, if the tattoos take other people's focus off of God then it is a sin.

BUT, if any tattoo we get can get others to ask us about it and it can be used as your personal testimony and by talking about it leads them to know Christ (it can be anything from a simple butterfly to an elaborate portrait of what ever tells your story) then it is not a sin.  Anything that we can do to get peoples attention to tell them about Christ is a good thing.  In our society, it takes a lot for people to notice anything! 

To sum up, if you are getting a tattoo make sure you have Christ in mind and how the tattoo can affect yourself and others and what it will do not only to your spiritual walk and to your character as a Christian, but what it will do to/for others (especially non-believers).