Hi, 

I am curious about how the thumbnail of a devlog post has been chosen when shared on Twitter. 

This is a devlog we made for our game:
 https://notapixel.itch.io/superstarfighter/devlog/93785/v050-new-sports-localization-improved-physics

And when shared on Twitter (see here https://twitter.com/notapixelstudio/status/1159134980724940801) 

An image that is not in the devlog (but it is in the game page) has been chosen... why? how? 

Thank you guys for the effort you put here :) 

I believe that's because a devlog is associated with a game, it uses the promotional image that can be found here:

Edit Game > Metadata > Promo Images > Social Media Image

Assuming you have not set your own custom image, maybe it's grabbing a random image from the screenshot uploads as a "default" ?