Praise be to Allah.
Tattooing is haraam (prohibited) and the one who does it is cursed, because of the report narrated in al-Bukhaari (5937) and Muslim (2122) from Ibn ‘Umar (may Allah be pleased with him), according to which the Messenger of Allah (blessings and peace of Allah be upon him) said: “May Allah curse the one who adds hair extensions and the one for whom it is done, and the one who does tattoos and the one for whom they are done.”
It is permissible for a person to tattoo some part of his body, if that tattooing is done to correct some defect or treat some sickness, and there is no permissible way to treat the sickness or correct the defect.
This is indicated by the report narrated by Ahmad (3945) from Ibn Mas‘ood (may Allah be pleased with him), who said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) forbid plucking the eyebrows, filing the teeth, adding hair extensions and doing tattoos, except by way of treatment. Ahmad Shaakir said: Its isnad is saheeh.
Ash-Shawkaani (may Allah have mercy on him) said: The apparent meaning of the phrase “except by way of treatment” indicates that the prohibition mentioned only applies if these things are done for the purpose of beautification, not when they are done for the purpose of treating sickness and problems; in that case, they are not forbidden.
End quote from Nayl al-Awtaar (6/229).
It says in al-Mawsoo‘ah al-Fiqhiyyah (43/158): The majority of fuqaha’ are of the view that tattooing is haraam, because of the saheeh hadiths which say that the woman who does tattoos and the one for whom they are done are cursed, such as the hadith of Ibn ‘Umar (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) cursed the woman who does hair extensions and the one for whom they are done, and the woman who does tattoos and the one for whom they are done.
Some fuqaha’ made an exception from this prohibition in the case of tattoos when tattooing is done as the only way of treating sickness; in that case it is permissible, because necessities make forbidden things permissible. End quote.
The way to treat cracks in the feet is by finding out the cause, whether it is a lack of vitamins or something else. It is possible to improve their appearance by using creams and henna.
It is not permissible to use tattooing for that purpose, because this is something that is done for the purpose of beautification, and having cracks in the feet does not reach the level of being a defect or deformity, and is not regarded as a sickness. Even if we assume that it is a kind of sickness, this is not a remedy for this sickness. Moreover, tattooing is not the only remedy for this problem; rather it may be treated by other, permissible means, and one cannot commit something that is a major sin for that purpose.
Based on that, it is not permissible to do tattoos to conceal cracks in the feet.
And Allah knows best.