Last time I heard of rich text it was a proprietary Microsoft format used by wordpad. So, I am just a little concerned the article was not entirely accurate or there is an issue in translation. Email in Thunderbird can be either text or HTML, but that has been an option for the last 20 years. Generally email is sent in HTML format and it is probably the most common format for email bodies now. (plain text ruled in the 1990s when Microsoft outlook was plain text only. Fundamentally you can insert an image into any email you write, either through drag and drop of the image file into the body of the email, paste if the image in one the clipboard (you have copied it) or through the insert image icon on the toolbar.