Microstock photography, also known as micropayment photography, is a part of the stock photography industry. ... A number of microstock sites also sell vector art, and some sell audio (music) files, Flash animations and video as well as images (Microstock photography - Wikipedia).
Microstock agencies have huge online stocks of images from various contributors. Usually they sell licenses for image usage, and the photographer keeps the copyrights.
There are 2 main classification for images:
No recognizable people or commercial logos, or location with other limitations.
These images can be used to any commercial use.
"news worthy", images with recognizable people, logos, etc.…
Photo manipulation is not allowed, non-manipulative pos processing is OK
These images can be used by newspapers, blogs or any other non-commercial use
Editorial images do sell
The best-selling images of the microstock agencies are generic commercially oriented photos. Also, this industry, in the last few years, seems to be over saturated, and photographers income is in decline.
However, travel photos do sell, and produce some income.
Shutterstock was the leader of microstock agencies. On 26/05/20 Shutterstock announce an update in the earning structure, that cuts down dramatically the earnings for contributors. This step could mark the end of microstock industry.
I started selling on various agencies in 2013. Slowly building my portfolio on some agencies, with some success (and income).
Shutterstock was by far the my best earner (due to highest volume, they did not pay more per download). They earning change is dramatic to me. I decided to disable my sales there.
I still sell on other agencies.
Links to agencies and my portfolio