It is narrated that ‘Abd-Allaah ibn Mas’ood (may Allah be pleased with him) said: “May Allah curse the women who do tattoos and those for whom tattoos are done, those who pluck their eyebrows and those who file their teeth for the purpose of beautification and alter the creation of Allah.” (al-Bukhaari, al-Libaas, 5587; Muslim, al-Libaas, 5538).
It was narrated that Abu Juhayfah (may Allah be pleased with him) said: “The Prophet (peace and blessings of Allah be upon him) cursed the one who does tattoos, the one who has a tattoo done, the one who consumes riba (usury or interest) and the one who pays it, and he forbade the price of a dog and the earnings of a prostitute, and he cursed the image-makers.” (Narrated by al-Bukhaari, 5032).
A person can become a Muslim even if they have had tattoos done to them or if they were involved in tattooing.
You can do not have to remove them in order to become a Muslim.
The curse is not permanent but it is lifted when a person sincerely repents to Allah for this mistake.
So one should not feel doomed because they committed this mistake when they were in days of Jahilia (ignorance).
You can pray, do wudhu and do all your Islamic duties even if you have tattoos.
This is because Islam erases all sins that a non Muslim committed before he or she became a Muslim.
As for things which are not permanent such as henna painting on women's hands , feet and so on, they are not regarded as tattoos and are allowed in Islam.
Many times I received e-mails from non Muslims wishing to become Muslim and from New Muslims who had tattoos done when they were not Muslim.