https://enviragallery.com/how-to-get-around-the-new-wordpress-image-compression-settings/

There are also plugins that will do this, for example https://www.ads-software.com/plugins/disable-jpeg-compression/

I’ll try editing function.php. I can’t find function.php (perhaps this is a typo). I do see functions.php under wp-includes.

I suspect I can edit functions.php via WordPress … Appearance -> Editor -> Theme Functions.

DO NOT EDIT CORE WORDPRESS FILES!

Functions.php is a file in your theme, which you should also not edit. You want to create a child theme and put it in the child’s functions.php.

Is there a web page that describes creating a child theme?

It would be good if WordPress provided a simple way to disable compression.

See https://www.electronicsiseasy.com/ -> Errata -> Clarifications.