The images are being automatically optimised by the pagespeed module
The PageSpeed Modules, mod_pagespeedmod_pagespeed and ngx_pagespeedngx_pagespeed, are open-source webserver modules that optimize your site automatically.
To confirm this load your site with the ?PageSpeed=off
switch at the end of the url, this will disable manually disable pagespeed for that url.
This means you have installed the pagespeed module on your apache / niginx web service and have enabled the image optimsation options.
You should check your server configuration and pagespeed module setup to change the way you want to optimise images.
Here is a link to the offical documentation https://www.modpagespeed.com/doc/filter-image-optimize