They claim to provide photos for universal use under a perpetual no-royalty no-attribution license.
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, worldwide 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.
Does this constitute a valid license ?
How, if challenged, can I use it to prove that I'm using a given photo downloaded from unsplash under license if the photo was later removed or the service was shut down ?