Thumbnail Blaster will help you easily publish your generated Thumbnails directly on your YouTube video.
In case it fails to publish, there are a few things that you need to try:
First, please visit: https://www.youtube.com/features?nv=1
Make sure that Custom thumbnails feature is enabled. If it is not enabled, enable it by phone verifying your YouTube account.
If you do have Custom Thumbnails enabled and it still fails to publish, try to make sure that you do not have set any custom Client ID/Secret. Please visit: https://app.blasteronline.com/thumbnailblaster/settings
Make sure both fields are clear (empty). In case the Client Id/Secret is not empty, simply remove all data and hit SAVE API.
After you do this, you will need to go back to YouTube accounts section of Thumbnail Blaster and REMOVE your accounts, then re-authorize them.
In case this still fails, simply try to remove your account and re-authorize it in Thumbnail Blaster.
This should fix your problems. In case you cannot publish Thumbnails, what we recommend is simply to download the JPG image that you generated, and change it manually on your video by going on YouTube and clicking on EDIT on your video.