How thumb image on video title
You thumb image on video title by just one condition when yours account is verified
You verified by clicking on custom thumb a"learn more" msg appear click on it
Scroll down click on verified Fill the blanks
After some time account is verified
YouTube studio app link:-
https://play.google.com/store/apps/de...
Thumb nail make app link:-
https://play.google.com/store/apps/de...