In my case there isn´t such an element in the list?
Welcome to our forum.
Displaying custom thumbnail is a little tricky but it easy once you understand, go each of your post (which post you want to display via post grid) you will see the meta field settings for post grid, please see there is an input field “Custom thumbnail source” put there thumbnail image source(should be image url)
Once you done for all post you need to display cusotm thumbnail then back to post grid settings,
Layout settings > Media source
you will see there are media source with checkbox at left, drag and sort “Custom Thumbnail” at top and checked the checkbox at left, and then save/update post grid.
you are done, just see the post grid, you will see feature image will replace with the custom thumbnail.
Hope you understand.
let me know for more help.