Page summary gets pageimage information from MW action API.
This is implemented in this extension:
https://www.mediawiki.org/wiki/Extension:PageImages
We need to add this extension as a dependency to our extension:
https://www.mediawiki.org/wiki/Manual:Extension_registration -> Requirements (dependencies)
Then we can fetch the required information for summary using this function:
https://github.com/wikimedia/mediawiki-extensions-PageImages/blob/master/includes/PageImages.php#L268