I usually use Fotor, and I really like it for its lightweight Photoshop-like functionality and it’s convenient for making youtube thumbnails.

First select your favorite thumbnail in the templates. (you can also add your own pictures in the computer)

You can choose any effect to make your picture look better

You can also transform fonts, add shadows

And change the basic color of the template

Add stickers and change colors…

Just have a try and you will love it !

Hope my answer helped.