
I solved this problem movie by movie: On the present thumbnail, click on get info/click on Get Artwork/drag and drop from Safari de movie image (search for it on Safari or Chrome)
It was tedious but at the end the result was better than it was before. I still have more movies to change the thumbnail.
On home movies, I did a screenshot of the thumbnail and edited it, adding a text box and wrote the title. Save it. It goes to "Recent" on the finder. Drag and Drop on the "Get Info", "Artwork".
For both solutions, after saving the drag and drop image, you need to click on the thumbnail again, it will open, then click the back arrow on the top left, and the edit will be there.
[Edited by Moderator]