I use Gmail, when I receive certain emails with embedded photos I can see them OK on Gmail's site but they are blank rectangles when I read them on Thunderbird.
When certain people (not everybody) send me emails with embedded photos, and I try to read them on Thunderbird, the photos come out as blank rectangles and they are converted to jpg attachments with funny names. Yet if I read the same emails on the internet at Gmail, they are perfect.
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Maybe they are including images that contain remote content. Remote content is blocked by default to protect your privacy as a security measure. https://support.mozilla.org/en-US/kb/remote-content-in-messages
I'm sure you're correct, but the photos in this specific situation are not from any remote source.