To add custom thumbnails to videos and enable other advanced Youtube features you need to verify you Youtube account. To do this:

Login to your Youtube accept

1. Go to https://www.youtube.com/verify (or find the ‘verify’ link within your account)
2. Choose either voice message or text and enter the phone number
3. You should instantly receive a call or text with a verification number
4. Once verified you can then edit the video with a custom thumbnail image