NYPL Image Archive Free and Online

Through the NYT, I find that the New York Public Library has made hundreds of thousands of images in its archival book collection freely available over the internet. I did a search for "India" and found this image, titled "Tattoo of the Sikhs at Fort Lister." These are Sikh soldiers stationed in Malawi in the late 19th century:

(Click on it to see the original context.)
QUESTION: Why the word "tattoo"? What does this image have to do with tattoos?