I created a signature (including an image) by writing what I wanted in Thunderbird email, then highlighting the image, then clicking Insert, then clicking html, then highlighting the raw code, then inserting into the Tools/Account Settings and pasting it into the box for Signature Text (also put a check mark in the "Use html" box). I only want to use the signature on certain emails. I thought that if I unchecked the "Use html" box after writing an email where I needed the signature, that on subsequent emails it would not appear. That doesn't work. What happens is that the html text (but not the image) still appears. Why is that?