Slow Scan television (SSTV) is a picture transmission method used mainly by radioamateurs, to transmit and receive static pictures via radio. https://en.wikipedia.org/wiki/Slow-scan_television
SSTV images are periodically transmitted from the International Space Station and other ARISS supported satellites.
To find announcements about transmissions, follow dedicated official ARISS blog:
Periodically ARISS dedicates special awards for SSTV events, here you’ll find any information:
After receiving,you can submit your decoded images and receprion reports in SSTV gallery:
For beginners, but not only, here is an AMSAT page very helpful:
Here you can find An SSTV decoder setup for Raspbian