Do you mean you want to incorporate the image into your message, like old-fashioned pre-printed stationery? There is an extension for that, but I haven't tried it myself, so I don't know how easy/difficult it is: https://addons.mozilla.org/thunderbird/addon/stationery/ Or if you only want this to be visible to you as part of your Thunderbird interface, maybe there's a theme generator or other handy extension for that? If it needs to be created from scratch, could you create a mock-up of where it would go? I mean, capture a screen shot of the places it should be inserted, and box those or add an arrow or otherwise explain where it needs to fit.