A rookie question- what key is the select key to make a thumbnail after creating something? It says push select, but I can't figure out where or what that is, Any help is appreciated.
The main UI has a camera icon at the bottom. That gives you a screenshot at medium resolution from any PoV you want. For 4K screenshots, you can use the Supershot plugin.
If you're talking about the "preview" that shows while saving a scene, or a look or a preset. There is no such thing as selecting the image. You're just "shooting" that thumbnail yourself in the scene when you save.
If you want to customize it, simply overwrite the jpg file in the folder manually with a thumbnail of yours.