Mosaic artists are often asked to render in mosaic the work of other artists. I have made mosaics based on the work of Diego de Velasquez, Van Gogh, ancient Israeli, Roman, Greek or Egyptian images and annonymous children's drawings bought in a thrift shop. No problems there. But when I wanted to incorporate a quote from the writer Barry Lopez or copy a photo of an Algerian village by George Steinmetz I got written permission to do this. The same went for the photo of a Russian railroad worker my son, David Gala, took on the train from Vladivostok to Moscow.
The general rule of needing copyright permission is the date of the death of the artist plus 70 years. So if the artist you want to copy died after 1945 you need permission from her or his estate, in writing. Take a look at the section on my website called "Inspired By" to see mosaic renditions of paintings and photos.