When I user ThunderBird, I usually like to add signature with words and a picture,but I found this is not support. It do dispointed me. So I think if it is possible to take an advice to deveopers. May be it will be supported in the future.
I would like to add to this that Thunderbird add regular formatting. My signature is two lines and I can't find a way for it to show up as such WITH the correct formatting. i.e., font and text size.
What kind of formatting do you mean by "regular formatting"? Perhaps you mean HTML formatting? (signature is two lines, WITH, etc.) It is explained in the official FAQ presented by @MattAuSupport .