When I make thumbnails for my videos I always use an image I get out from the video itself.
This can be done very easily (but might need some patience and practice).
Watch your video and make up your mind which part of the video would make a great thumbnail.
It is recommended to make a screenshot out of the first few minutes (if possible seconds) of your video, so people see that your thumbnail didn't lie to them!
Jump in your video to a part that is at least 10-20 seconds before the part you want to capture.
Switch to fullscreen mode and start the video. (The video will show some stuff like “to exit fullscreen...”. That's why we have to jump a bit before the part you want to capture, because we don't want this on our thumbnail.)
Now you need some patience and luck to capture the correct frame!
To make a screenshot hit the “print” (sometimes “prt”) key on your keyboard. You can only hit it once, or you will override your previous screenshot!
Stop the video and open the program paint or another picture editor.
In the picture editor either, right click and paste or hit “control + v” to paste the screenshot into paint.
If you got what you wanted you can safe it and go back to Part 3 of my training.
That's it for now.
If you want to know more about thumbnails or maybe about Youtube, let me know and if it's something I can help you with I will create more trainings for this topic.
Was there anything difficult to understand?
Let me know and I will add pictures or change descriptions.
Yours,
Moritz