Step 1
right click on your image and chose "copy link image"
98f83e97-870f-4442-b439-1b24a50a9c0a-image.png
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"
image_2022-01-09_225145.png
Step 3
You will see a piece of code covered in blue like this
image_2022-01-09_223946.png
double left click on the line "https://........" and delete it
1583e4c4-4a2c-4d4b-9269-9ffd7b84a303-image.png
paste the link of the image you copyed in step 1 between " " like this then press Enter
f20f50d5-f160-45cc-8caf-09b5b438cd26-image.png
the thumbnail you choose will look like this
35f34cac-23bc-40a5-8ee3-51fbb0eb0c17-image.png
Step 4
Ctrl + D and find the image you want to change by left arrow and right arrow and click Done
image_2022-01-09_225620.png
image_2022-01-09_225828.png
Result
image_2022-01-09_230012.png
Goodluck and sorry cuz my English is bad😅