The key importance to a thumbnail would be to make sure there's a clear, relevant picture for the video so viewers will know what the video is about. Another thing to consider, which you'll see a lot of YouTubers do, is to create branded thumbnails to put on all of your videos, that way they all look similar and uniform, which will give your channel a very clean look. If you have a lot of videos in the same place, it might be good to brand them with similar styles and colors.
I don't think thumbnails are something people will go out of their way to notice if done right, but if you have really weird or bizarre pictures for thumbnails, you can guarantee people will notice. So it's probably best to invest a little effort into developing worthwhile ones just in case.