To insert an image into your signature in Mozilla Thunderbird:
Start with a new, empty message using HTML formatting.
In the message's body, create your signature the way you want it to look.
Insert the image.
Create a new message in Mozilla Thunderbird.
Position the cursor where you want the image to appear in the body of the email.
Select Insert | Image from the menu.
Use the Choose File... selector to locate and open the desired graphic.
o If your image is larger than some 640x640 pixels, consider shrinking it to more handy proportions.
Type a short textual description of the image under Alternate text:.
o This text will appear in the plain text version of your email. People who choose to see only this version can still get an idea of where the image — which is still available as an attachment — appears.
Continue editing your message.
Go to the image insertion dialog's Link tab to link your image to a web page.
* Select File | Save As | File... from the message's menu.
Make sure HTML Files is selected under Format:.
Enter "signature.html" (or something similar) under Save As:.
Close the message window, not saving the draft.
Select Tools | Account Settings... from the Mozilla Thunderbird menu.
For all desired accounts:
* Go to the account's root item.
* Make sure Attach this signature: is checked.
* Click Choose....
* Find and highlight the "signature.html" file just created.
* Click Open.