How do I reply in HTML to a message received in plain text?

I have a signature with a tiny graphic in it, so it’s HTML. With most messages I can just click Reply and the sig is attached. But if someone sends me a plain text message, and I Reply, I get the sig without the graphic. I can then convert the reply form to HTML but the graphic does not reappear, so I’d like to “shift-click or something” to force the original opening of the reply form to be in HTML, so that the graphic comes with it.

thanks for your suggestion. I will add the issue to our official feature request list so our programmers will be able to discuss about the best satisfying solution.