Meetup added a new group header graphic in their late 2017 redesign. This shows up when your group is shared on social media. You can replace the default image (which might look terrible and be badly cropped) with any image.
A reminder that meetup growth is STRONGLY correlated to scheduling monthly events. After being inactive for several years @karenb and team restarted the group in January 2016. And ever since growth has skyrocketed. LA Tech4Good is now the fifth largest group!