Hello,
An issue with using Webp images (the source file) has been identified in the Grid format for Moodle 4.0+. Please don't upload Webp images for the sections until I've released a fix - they will break your course and they'll be nothing you can do bar restore from a backup! If you want to know more and test the patch, then please see: https://github.com/gjb2048/moodle-format_grid/issues/190.
Gareth