You can try it, but many email users seems to have trouble seeing such images.

I just copied the address given in a link to an image (your avatar in your posting), inserted it as in image into a test message, cleared the "attach this image to the message" check box, and the received message had just the link, showing the image. Nothing embedded or inlined.

  …
  
    Here is a button

    

    <img moz-do-not-send="true"
src="https://secure.gravatar.com/avatar/50b085d124fee702b3864b250a2d5c08?s=48&d=https%3A//support.cdn.mozilla.net/static/sumo/img/avatar.png"
      alt="" height="48" width="48">

    

    Is it "inlined"? 

    
-- 

      Chris
  

I don't know if it makes any difference, but I used a slightly different method: right-click the avatar, Copy Image, then paste (Ctrl+V) into the body of an HTML message. The html code is shown in the picture.

When the message is received, the body shows an empty box, and a banner below the header: 'To protect your privacy, TB has blocked remote content...' as usual, and then by clicking the Options button the image is loaded.

Maybe the trick is to not just enter the URL of the image, but the Image Location?