I believe this provides a solution, but from my tests it is not necessary to modify the signature. My problem was that I wanted to include embedded images in my email and non-iOS email readers were receiving the images as attachments. By default, iOS seems to send mail as plain text (as has been stated) and images are therefore sent as attachments rather than being embedded in the text of the mail.
I tried the approach suggested by bnb2235 and found that it works. I copied and pasted a random piece of HTML text from a web page viewed in Safari and into my email signature on my iPhone 6. After this, images included in my mail were embedded in the text of email sent from my iPhone when viewed in my Outlook mail reader on Windows.
But I then found that it is not necessary to modify the signature at all. All that is needed is to alter the font of any text that is included in the body of the email (e.g., long press on the text and change the font to italics or bold). It seems that this causes the iOS mailer to convert the email format to HTML which enables it to embed the images in the body of the email. After doing this, the images were embedded as I wanted. Of course, using HTML in your signature will also have this effect and helps to automate the process, but it is not necessary.
Thanks to all for helping find a solution to this problem!