How to Add a Picture to Your Thunderbird Signature

Step 1: Compose a new, empty using HTML formatting. If an existing signature shows up, delete everything in the body of the message.
Then build the signature to your liking, including any text that should be included.

Step 2: Place the cursor on a new line and select Insert > Image to add a picture to the body.

Step 3: Select Choose File and pick an image from your computer.

Step 4: Select OK to place the image.

Step 5: Drag the handles to resize the image if needed.

Step 6: Select File > Save As > File. If you don’t see the menu bar, press the Alt key.

Step 7: Confirm that the Format or Save as type option is set to HTML. 

Step 8: Choose a name for the file and save it. Close the new message you created, and discard the draft.
Then, Select Tools > Account Settings from the menu bar.

Step 9: Select the email address in the left pane for any account that should use the custom email signature.
And at the bottom of the window, select the check box next to Attach the signature from a file instead (text, HTML, or image).

Step 10: Select Choose.

Step 11: Navigate to the HTML file you saved previously, and select Open. Then Close the Account Settings window.
After that When you start a new email, the signature appears automatically.

Credit : https://www.lifewire.com/add-image-to-thunderbird-signature-1173166