What is Unsplash?
All photos published on Unsplash are licensed under Creative Commons Zero which
means you can copy, modify, distribute and use the photos for free, including
commercial purposes, without asking permission from or providing attribution to
the photographer or Unsplash.

For more information on Unsplash please visit this page:
https://unsplash.com/help


Unsplash API

You need first register on Unsplash https://unsplash.com/developers

Then you need to create application https://unsplash.com/oauth/applications/new

Enter title and description, leave Redirect URI empty. 
Permission as they are by default.

After application is created copy Application ID inside Drupal module settings
page.

Now, by default, you can make up to 50 requests per hour while application is
in development mode. You can increase this to 5000 per hour, but you need to
send screenshot of your app. You can also mention that is Drupal module in
question (backend access only).


More about API limits and your API key please visit this page
https://unsplash.com/documentation