Question

Yes! I have seen how to do but I am doing 1st Time and don't have computer also.
And.....
1. First I clicked on Custom Thumbnail
2. Then there was written....
TO ADD CUSTOM THUMBNAIL, VERIFY YOUR ACCOUNT ON YOUTUBE ON YOUR COMPUTER.
3. Then there were two options: CANCEL and LEARN MORE
4. I clicked on Learn More... There was a video...In that when they clicked on Custom Thumbnail so images came but not in Mine's one​

Answer 1

Answer:

i think there was a bug there. maybe that is why it is coming like that