I'm experiencing the same issue and it appears to be related to using the "dark, including messages" color theme in AquaMail.
If I use that setting, none of the images are visible in emails from YouTube. If I change the color theme to any of the other three options (like "dark, with light messages"), the images display fine.
To temporarily see the images while the YouTube email is open and when using "dark, including messages", select view > enable white background.
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