I'm very interested in your journey through our docs. I'm wondering if you missed the Node.js samples repository completely, or if we have not promoted it well enough?
All of our samples for Node.js are listed on this page - Media Services v3 samples
Most all of the encoding samples demonstrate how to generate a PNG or JPG thumbnail.
I would recommend most customers to use the Content Aware Encoding preset if they are not familiar with encoding. Content aware encoding with H264 - that one generates a default thumbnail as well.
For more advanced thumbnails - see the sprite VTT sample
Create a thumbnail sprite (VTT) in JPG format - Shows how to generate a VTT Sprite Thumbnail in JPG format and how to set the columns and number of images. This also shows a speed encoding mode in H264 for a 720P layer.