sudo find /srv/lemmy/example.com/volumes/pictrs/files -type f -ctime -1 -exec rm {} \;

• sudo: run as root
• find /srv/lemmy/example.com/volumes/pictrs/files -type f: find files (f) in directory
• -ctime -1: which have been created in the last day
• -exec rm {} ; execute the command rm (remove) on each of them