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