You want public domain images because they are free to use. Images (photos, infographics, clip art, etc.) are protected by copyright laws and they are not free for your use in your blog post or on your web page unless the creator has released their copyright, or the copyright protections do not apply for some reason.
For instance, federal government images are not protected by copyright and you are free to use images found at sites like NASA and the CDC.
Publicity Stills: Another Source of Public Domain ImagesAnother great find for public domain images: publicity stills that are in the public domain. Like Fred Astaire dancing in promotion of the film "Daddy Long Legs" above, of the image below of Elvis Presley promoting "Jailhouse Rock."
film stills" or "publicity photos." Lots to choose from - with famous faces and people doing things other than dancing.
These Hollywood Golden Era movie stills are free from copyright protection and in the public domain because they were published in the United States between 1923 and 1963 and any copyright was not renewed. However, publicity stills by definition are generated for publicity and many, like news releases, are disseminated for widespread use with a release of copyright in order to promote an actor or a movie or a TV show.
You need to check before using newer images, however. For details on more recent imagery, visit CreativeClearance. Newer film stills may retain their copyright and you'll need to pay for their use in your publication (and a blog is an online publication).
From Wikimedia Commons:
As explained by Additional source information: This is a publicity photo taken to promote a film actor. As stated by film production expert Eve Light Honathaner in The Complete Film Production Handbook, (Focal Press, 2001 p. 211.): "Publicity photos (star headshots) have traditionally not been copyrighted. Since they are disseminated to the public, they are generally considered public domain, and therefore clearance by the studio that produced them is not necessary."