In the publishing business, in the 18 hundreds, books were sold by mail order. People got a page with titles in the mail, or through a representative, you traveled the land. A book was read by its cover, so to say.
They tracked which books sold best. And when they did not sell well, what did they do? Throw it away? no! They changed the title of the book in the catalogue, until it sold. That is where copywriting started...
Now over to the youtube age. When you google something - which video do you click first? The one with the attractive thumbnail, right? So the thumbnail is the new copywriting title. Keep it, it will get stale. Change it regularly, you get a chance of getting new attention.
Thumbnail Blaster does exactly that - and makes it really easy, as well. Thumbnail Blaster: