I've taken over a site from another developer. They used advanced custom fields plugin to create custom content type, and unfortunately didn't use the "featured image" for images, but rather a new custom field.
How can I access that field to be used in "media" for showing the image?
Welcome to our forum.
I am not sure if you using pro version of post grid, there is element available for ACF fields, please see the screenshots, you don't need to use media element in this case