Thanks for your patience regarding this issue.
Fixes on macOS for PHP 5.6.39 and PHP 7.4.1 are now available. GD has been re-added to PHP 5.6.39 and the dynamic libraries for ImageMagick in PHP 7.4.1 have been updated so
brew install-ing dependencies is no longer needed.
Solution: If you’re running into this issue, and you’re on macOS with PHP 5.6 or 7.4, see How can I manually update downloadable Lightning Services? to fix this issue.
Note: PHP 7.3.5 has also been updated on our end, but it is a bundled service and not downloadable. We will include a fix for GD missing in PHP 7.3.5 in the next stable update.
In the meantime, you can switch to PHP 7.4 or 5.6 if you’re running into issues with image thumbnail generation.