everyone has clearly been going around this topic as the answer is a mess that can not be offered as do this get this.

HOwever. common causes are.

Anti virus products. Somtime that block image libraries, so if someone sends 5 or 10 images from what is essentially the same web page they block it. I have no idea why.

Anti virus product. Blocking the image source as a "threat" again I have no idea what the threats are. sometime they are legitimate and sometime reflective of the poor quality of the product.

Mail from Microsoft mail product that is incorrectly encoded. Mictisift think the specification is determined one way Mozilla think the reverse and when a specification says "may" then neither are wrong. but it is damned inconvenient.

Mai from web scripts on web pages often make some very stupid encoding decisions. This is largely because most web developers have an exceedingly poor understanding of mime encoding, so the send images as binary objects or something equally silly and Thunderbird calls them on it. as doing so is also where the risks come from as far as malware in email go.

That is just a generic list of possibilities. it in in no way exhaustive or even necessarily applicable. But you are talking about issues that are quite technical and I have no technical information (like the original email) on which to base my guesses and assumptions.

Another possibility is that your emails are being viewed with "Simple" HTML instead of "Original" HTML. I was able to restore the missing pictures in my version (68.3.1) of Thunderbird like this: In the Menu Bar: View > Message Body As > Original HTML.