I'd like to send out an email message that has an IMG tag referencing
a GIF on my server so that I don't have to attach it to the message
itself and waste bandwidth. I've been trying to do so in Thunderbird,
but every time I add an IMG tag, it automatically attaches the GIF to
the email itself, reflected by a major increase in email size. I've
also tried attaching a simple HTML file with the IMG tag in body, but
then it comes into the inbox as an attachment regardless of the
setting.

For full payment, you will simply show me how to do, what is called in
common parlance, an "e-mail blast" in Thunderbird. When I send the
message, the size reflects only the text I send, and not the GIF, and
when I open it in Yahoo mail, the img is in the main body of the
email, and not an attachment.
Thanks,
Marc

hmmm...

I downloaded Thunderbird to just check this out. 

I knew how to do it in Outlook Express. Tools->Options->Send->(Mail
Sending Preferences) HTML Settings->Uncheck the "Send Pictures with
Messages" checkbox

And only your html code will be sent and not the images. 

But when I tried the same thing in Thunderbird,
Tools->Options->composition->HTML Options, I don't see any option to
change the image attachment setting.

May be it's not yet available in thunderbird or may be it's slightly
tricky. but if it is urgent, you can use outlook express, instead.

It's not too urget; I just wanted to get it working in Thunderbird. I
hope it's not impossible and that someone knows how it's done.
Thanks anyway, though.

I havent installed Thunderbird to try this myself, but try these steps.

From the message's menu, select Insert | Image.... 
Put the cursor in the Image Location: field. 
Paste image url. 
Add some alternative text that will appear in the email message if the
image you linked to cannot be accessed.
Click Advanced Edit.... 
Type "moz-do-not-send" under Attribute:. 
Enter "true" as the Value:. 
Click OK. 
Click OK again. 

There is also a screenshot-walkthrough available at
http://email.about.com/library/daily/et/blet092201_2.htm

Found the answer at about.com http://email.about.com/cs/mozillatips/qt/et092201.htm

That did the trick! Thanks!
But hmmm... Aaron, it looks like you're a commenter and not an answerer.
So I guess all I can do is say thank you very much!