According to Unsplash.com:
Unsplash was formed as the antithesis to the stock media experiences available at the time. Instead of vast libraries, licensed and presented for commercial buyers, we focused on pushing the impact of photography further than ever before by making original, high-resolution images available for anyone to use for anything.
See more: https://unsplash.com/manifesto
All photos published on Unsplash can be used for free. You can use them for commercial and noncommercial purposes. You do not need to ask permission from or provide credit to the photographer or Unsplash, although it is appreciated when possible.
More precisely, Unsplash grants you an irrevocable, nonexclusive copyright license to download, copy, modify, distribute, perform, and use photos from Unsplash for free, including for commercial purposes, without permission from or attributing the photographer or Unsplash. This license does not include the right to compile photos from Unsplash to replicate a similar or competing service.
See more here: https://unsplash.com/license