How to change default Memories photo in Apple Photo app
To change, click on the person who you want to modify,  then press "Select" and click on the photo you want to change it to. 

Then press the "Share" button and scroll across to where it says "Set Key Face." Press this and that photo will be the main photo for that person.

On iOS 12 - the thumbnail for memories is gone, so perhaps that will happen in Photos on macOS or the web interface to photos so you don't have to mess with curating the hidden attribute of a photo you want out of the "featured" image rotation. Hopefully adding new memories lets those photos you want to see rotate in as the main placeholder, but there's no direct way to choose that today.