Oh. Ha!
I'm not sure how detailed the phone interface is for changing video settings. I suspect it's similar to the desktop:
Quote:
Add or change video thumbnail
- Sign in to YouTube.
- Go to Creator Studio > Video Manager.
- Next to the video, click Edit.
- On the left of the preview screen, choose a thumbnail.
- Click Save changes.
You can also use the YouTube Studio app to add thumbnail to a video.
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More info on changing video thumbs:
https://support.google.com/youtube/answer/72431?hl=en