Step 1
right click on your image and chose "copy link image"

Step 2
go to https://www.youtube.com
press 3 keys ctrl + shift + C at the same time to open dev tool.
right click on any thumbnail image on Youtube to open a popup window, then chose "inspect element"

Step 3
You will see a piece of code covered in blue like this

double left click on the line "https://........" and delete it

paste the link of the image you copyed in step 1 between " " like this then press Enter

the thumbnail you choose will look like this

Step 4
Ctrl + D and find the image you want to change by left arrow and right arrow and click Done


Result

Goodluck and sorry cuz my English is bad

Hello,

I've got the same default image as you with the white letters... cannot change it...