Here are some of my favorites from my comic for now. I like to diversify them by highlighting different characters with different expressions. Though, next set of episodes will mostly feature the dog in the thumbnail.
I get a little sad when I check out a new series, and the episodes don't have unique thumbnails. They're easy to make. Just take a panel from each episode and cut it on one of the faces. They're are the easiest to read when small.
It's fun to look back at your episode list and try to figure out the story through thumbnails. they're like a mini recap.
Where the thumbnails come from: https://tapas.io/series/Pembrokes-Tale/info