Using thumbnail instead of post image

Hello,

I am trying to create a post grid where I am using a custom thumbnail instead of the post featured image. When I create a layout and "Thumbnail" from the layout editor and also select my custom image in the post, my custom image does not display on the post grid. If I use the "Media" and create a post featured image, it will display. But I don't want to use the featured image on my post.

Thank you.