Though using free images from Unsplash, Pixabay and other sources in personal website is common and however using in commercials or themes is restricted due to their copy right restrictions. Because my sites are personal and I have been using images from Unsplash and Pixabay for a while.
Most free content is either classified as being in the public domain, meaning it is old enough to no longer be subject to copyright laws; copyright-free (also sometimes referred to as copyleft); licensed with GNU Free Documentation; or under one of the Creative Commons licenses for use and redistribution.WordPress Support
Creative Common (CC) Search
Creative Commons, a non profit organization “enables the sharing and use of creativity and knowledge through free legal tools.” The Creative Commons Search page includes searches “within Wikimedia Commons, YouTube, and Open Clip Art Library, among others” which can be used to find content that is “licensed for commercial use as well as content that may be modified”. In April 2921, WordPress acquired CC Search which has been now available as Openverse.
The CC Search now redirects to WordPress.org/Openverse and its users can now “enjoy an improved experience, including access to images from new sources like StockSnap, along with new Meta Search providers like EDUimages and Images of Empowerment“. It’s is reportedly planned to integrate openverse to WordPress media library.
Sarah Gooding has covered CC search and Openverse in WP Tavern in more detail.