1. You can't really include an image in your signature without making your email HTML. You can attach images without using HTML, but then you really have no control over how (or if) those images are displayed. So if you like plaintext (and I personally applaud you for being a responsible netizen and using plaintext and not HTML), then you really can't do this.
2. The other questions you ask are basically moot, but if you really wanted to make your messages HTML from now on, you could just attach the file to every email and include it in your sig or you could put it somewhere universally accessible (like at a web address) and put the address of the image in HTML in your sig something like <img href="http://www.place.net/myimage.jpg" />. I do not believe that FM's webmail interface would let you do the first option (and that's probably a good thing), but I'm sure you could use the second one if you have a place to host the image (and I might not use FM's file storage for this task if you're sending messages with this image url in them to a lot of people -- you could end up using a lot of your bandwidth quota depending on what kind of volume of mail we're talking about).