It would also be an improvement if they were displayed in both the Composition window and when reading a received e-mail so they fit on the screen rather than only seeing the top left corner.
If you'd like to try Installing an Add-on in Thunderbird https://support.mozilla.org/en-US/kb/installing-addon-thunderbird Enter the add-on's name Shrunked Image Resize or enough of it to limit the search results returned to a manageable number and press Enter or click the magnifying glass icon.