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.
Wšykne wótegrona (2)
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.