How do I pick a different thumbnail (screenshots included)

I have the theme ‘Goodtheme’ and I don’t think there is a ‘featured image’ option. Does it take the thumbnail randomly?

I need to phone to be the thumbnail, not the download button.

Screenshots:

https://gyazo.com/71dd6f55d913131f8f0e5c75efe6a24c.png

https://gyazo.com/aebef5e927fedff6c701a8c66f41ab84.png

Regards