First in order to enable you YouTube videos to be able to add custom thumbnails you must both verify your account & become a YouTube Partner. Both of these steps are simple.
Verifying Your Account
In order to verify your account you need to go to the “Video Manager” shown below.
Once you do that you need to select channel settings on the left bar. Once you do you will be shown a screen like below. You need to make sure your account is verified & you are a certified partner. Once you go through the process the little light for custom thumbnails will turn green.
After custom thunbnails is enabled we can now add it to one of our videos. To do so we need to go back to “Video Manager” and then select “Edit” on the video which we would like to add a thumbnail to. To the right of the video under the automatically generated thumbnails to select from there should now be a button enabled named “Custom Thumbnail”. Upload your photo and now that will be what people see before they play your video!
The best size to make your thumbnail would be 1280 x 720 pixels. This is so that no matter how large your video is displayed, the thumbnail will still remain nice and crisp.