Currently I have an HTML signature for my work account on Thunderbird desktop that contains some simple text formatting and a logo image, but pasting the same HTML into the signature setting in K-9 Mail just inserts the HTML as text into the email.
Additionally, an image signature would be perfect! I don't understand why it is so difficult to implement an HTML signature, since HTML emails are standard nowadays anyway. I like the Thunderbird app, otherwise I would have switched to an alternative.
There was an Guthub Issues #794 (and another Issues) about this topic back in 2015, but unfortunately it hasn't been implemented yet 😞