Headers

Hi! I’m new to WordPress and I need some help changing the header. On my admin page it says that I successfully changed the Header to the image that i prefer. But, when I try to send my site’s link to someone, the thumbnail still shows the old header which is unrelated to my site. for example, I’m trying to send the link to a friend on Facebook. The title of my website shows up correctly, but then below the URL a picture of a church appears (I’m using the Church Omega child theme). I would like the picture to be the same as the header on my website How do I change the header so that when I copy the URL and paste it into Facebook messenger it shows the new header i have chosen.

Thanks!