Stock Photos and Videos.
shutterstock API is a free Photography API that requires OAuth authentication, is served over HTTPS and has unconfirmed CORS support.
Details from api-reference.shutterstock.com.

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The shutterstock API is provided by Shutterstock API Reference (api-reference.shutterstock.com).
Yes. The shutterstock API is listed in the Photography category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The shutterstock API uses OAuth authentication, so you need to register for credentials before making requests.
CORS support for the shutterstock API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The shutterstock API is served over HTTPS, so it's safe to use in production.