I'd experiment. I'm not the most artistic but I find canva.com (there's a sticky thread about it in this section) is a load off my mind.
It's simple to use and it's free , you can create some great thumbnails.
I make videos giving out facts about movies in the horror and sci-fi genre so I try and use eye catching images, it's usually an image from the movie with the movie title splashed over the image.
I'm not saying they're perfect but I think they look decent especially compared to how they were before.
I'd give that a try.