Out of the blue almost, Well, about 2015 in fact,
it seems, came a New Image File Format:
“High Efficiency Image format” .heif & .heic
that miraculously is a third or even a half the file size of conventional JPG and GIF formats we are so used to with comparable-or even better- image quality :
It also ‘does’ a lot more and has more capabilities

Starting with  Phone 7 with IOS 11 or newer,  and contemporary  Android devices, this is the default format– and No Wonder. The ‘high efficiency’ achieved would normally be at the cost of heat and precious battery life  in portable devices due to the heavyweight software calculations involved.
But now, this process is embedded within the Silicon Chips  your device for far higher efficiency- Hardware. Tis is also generally true of Intel CPUs of ix 8th generation or higher (Since 2015 approx) 
More Here:

https://en.wikipedia.org/wiki/High_Efficiency_Image_File_Format

A comparable format for Video appeared about the same time, HEVC: High Efficiency VIDEO coding (more about that elsewhere) suffice to say I noticed without knowing why, a 50% DROP in  Face-time Bandwidth used if both ends  had at least an iPhone 7 or above :))

So your phone’s precious Storage is spared when taking still images- Or videos- Transferring them uses less of the main limiting resource on the Internet–Bandwidth, and everyone is happy, right?

Almost.
Built in to most newer devices is the ability to ‘down convert’ if sending to older formats. I assume SOME negotiation takes place but it’s relatively easy to convert formats ‘transparently, on-the-fly’ using the Silicon Chips I just mentioned, 
They are handed the job and hand it back Done.
The processing MUST be real time- albeit with some acceptable  latency-  else Converting, Streaming, Saving would impose unacceptable delays.
Minimal Buffering/Latency  is a side effect of Powerful Silicon.

How about we Linux users? I use Linux MINT TARA, based on Ubuntu 19 something,  all nice and up to date, but with a 10 year old CPU and no special graphics card GPU to help..;
No real-time stuff for me!
And sure enough no Thumbnail Displays of  .HEIF images in its native File Manager NEMO  nor edit them in Gimp, the “Graphical Image File Manipulation Program”, despite its name
etc. 

sudo apt-get  install libheif-examples
the command : “apropos heif”

Shows you now have:

heif-convert (1) – convert HEIC/HEIF image
heif-enc (1) – convert image to HEIC/HEIF
heif-info (1) – show information on HEIC/HEIF file
heif-thumbnailer (1) – create thumbnails from HEIC/HEIF files

Fun! It works, too. I shall Leave The Examples Of Batch Processing Up To The CLI Scripters : )

 

.. To be Continued …

 

 

For command line tools, add: