When I add an image to the site and share story to fc, if I decide to add another image to the site to update my story after I do that does the story I previously posted as a cast on fc get automatically updated or do i have to share the post to fc again to see the updated story?

I hope I come back as a rock in my next life.

Ohh that’s pretty convenient! 
Didn’t realize that must have been what you were talking about with the caching time

it will update automatically after a while (~10 minutes)