If you are seeing a rectangular outline box with a broken file greyed image in it:

This means the image contained remote content. By default, Thunderbird does not display remote content for security reasons. Please read info at link for more info:

• https://support.mozilla.org/en-US/kb/remote-content-in-messages

"If you are seeing a rectangular outline box with a broken file greyed image in it..." Nope! There is nothing there, just blank space.

I had the same problem with the newest update. Prior to updating, I saw all images. After, some messages showed pictures and some did not. I solved the problem by going into Thunderbird settings and then privacy. In the older version, I had some exceptions listed and still saw all pictures. After removing all the exceptions in the newest version, I can now see all pictures.