I just read this thread and it puzzles me. Then I wondered whether you are using HTML email or text-only email, because text-only emails don't have any embedded formatting or images in the text. Dragging and dropping images into the text composition area of a message works (on my computer) works when a message is composed with HTML formatting, but not in simple text mode.
If you haven't changed much from the default appearance of THunderbird, when you compose a message in HTML mode (kind of the "modern default"), there is a formatting bar (B I U...) at the top of the composition window. When you compose in simple-text mode, there is no formatting bar.
So four settings could affect your possibility to send images within the core of the text.
(Please note that I use Thunderbird in French, so menus may slightly differ from my translation.)
- Press on ALT to display menus, then select Tools –> Options.
– Select "Composition", then at the bottom right, click on "Sending Options"
– Make sure that the first check box (Send in text format when possible) is NOT checked.
– Also make sure that the person you want to send an email to is not part of the domains you have included in the list of "domains to which you only send simple-text messages".
– Then click OK to close all these message dialog boxes.
- Right-click on your account name (left side-panel of Thunderbird's main window) and select "Account Properties".
– Go to "Composition and Addressing" (3rd sub-item)
– Check on the first box: "Compose messages in HTML". Make sure it is checked.
- Third, if it doesn't work for a particular sender, check the contact itself in the address book to make sure you haven't flagged it as someone who accepts only text-only messages.
- And finally, a remote possibility: if you are replying to a message that was sent to you as a text-only message, I think that Thunderbird will convert it to an HTML message only if you add extra items like formatting (bold, italics, different fonts or font sizes, etc.) or accented characters. In such a case, it might be that you won't be able to drag and drop an image until you convert the message into an HTML one.
(I must say I don't know of the current status of this last item; as I write in French most of the time, all our messages have accented letters, hence are not text-only messages.)
See this page for reference (still applies today):
https://www.ghacks.net/2010/08/16/switc ... hunderbird