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.
BlueMail (for Android) has this feature, and it's helpful. When sending an email, it prompts the user to select original, medium, or small photo resolution.