https://unsplash.com/licenseAll 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.
The format goes:
Wallpaper's thumbnail (linked to the 1920x1200 image)
Wallpaper name
URL of the original photo
Information about the photo, provided by the photographer or by me in that order of preference
Photographer's name
(The last two are reversed for some wallpapers.)

Angel
https://unsplash.com/photos/2mjl2uvz9ic
Metal abstract, Based, Switzerland
Ricardo Gomez

Big_Sur
https://unsplash.com/photos/znekTMMoBrM
Big Sur, California, USA
Jesse Collins

Clouds
https://unsplash.com/photos/Yx0sVL2H8pA
Clouds at sunset
Zhao Chen

Godafoss_Iceland
https://unsplash.com/photos/R8OgEsHVOhA
Mahkeo

Landeyjahofn_Iceland
https://unsplash.com/photos/Ef7C4bPDL4A
Golden dust and light at an Icelandic beach
Mahkeo

Landmannalaugar_Iceland
https://unsplash.com/photos/t07FAEn9wAA
Eroded mountains in Iceland
Jon Flobrant

Lladdwyn_Island
https://unsplash.com/photos/pMvIjMu9HSM
Twr Mawr Lighthouse on Lladdwyn Island in North Wales at sunset
James Eades

Rhino
https://unsplash.com/photos/hcjjoTawInU
Captureson

Silver_Falls
https://unsplash.com/photos/oPikeDMH0gA
Silver Falls State Park, Silverton, Oregon, USA
Eric Muhr

Sonoran_Desert
https://unsplash.com/photos/em7gIbjEL0I
NOAA (National Oceanic and Atmospheric Administration), US Government

Waves
https://unsplash.com/photos/OCzvgBqCJKY
Ocean waves
Silas Baisch