Firstly, I'm not a developer, mostly a website designer and CSS hacker.
I'll try to explain better:
I'm using a grid theme that creates a thumbnail out of the featured image to display as a post teaser. My previous theme would do the same, but would use the custom thumbnail that I created in the Thumbnail Setttings in the Media Library.
I imagine this is a simple change, maybe in the functions.php file?
I looked in the existing posts, but could not find a suitable answer. Feel free to link me to an existing thread discussing this.