"A thumbnail, for YouTube, is the cover still image seen before clicking a video. The intention is to draw the viewer in with a stand-out interesting image accompanied by an interesting title."
Source: Filmmaker /
Blender / 
Photoshop
Improving discoverability and SEO is a major point of importance for modern channels, among them is a catchy, interesting thumbnail that compells audiences to click on the video. I produce these for my own content or for other channels that commission it for me.
I have made many in total, here is a selection among the many.
