Hello @ElizabethO7 ,
If the video site's API is unable to provide the thumbnail desired, or if there's an issue with importing the particular video link, then the best workaround would be to generate your own thumbnail image to link the video to. This is how you'd get to "upload a custom video thumbnail."
There's 3rd party sites and other programs that can be utilized to also add a play button overlay on your image, prior to uploading and inserting into your email.
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