This is the still photo (ostensibly from the video) that is shown with the title and description in Google and other searches. On YouTube, you will often see what appears to be a bare-breasted woman in the small video thumbnail. This leads some to watch the video, not realizing that the video may have nothing to do with bare-breasted women. I’m not suggesting you do that, but the point is an appealing thumbnail gets a lot more views. Allowing a random default thumbnail to become the symbol for a video will bring fewer views. YouTube sometimes changes the rules on how this works, but it is something to study if you want many people to see the video.
Custom thumbnails are the key. Here are some tips: Use a close-up of a person’s face with eye contact. An emotional moment in the video is good. Use a frame or image that really reflects the content of the video. A brightly colored background draws attention and will make the thumbnail stand out. When there’s room, also use the company’s logo. Keep it in the bottom left of the screen so it doesn’t cover the screen. Avoid placing anything important in the lower right hand corner. When producing several videos for a company, use a consistently styled thumbnail on all videos. Make sure the thumbnails work on mobile phone screens as that’s where they will be seen most often…
Excerpted from Marketing With Digital Video.