How to use OS X HTML Mail signature on iOS Mail
There is a fairly easy work-around to this.  It requires that you setup your email signature in your Gmail account online.  You can include an image, which means it gets uploaded to your Google account, with a permanent reference to it.  Once you've setup your signature in Gmail, just send yourself an email from the online account.  Open this email up on your phone.  Now select the content of the email, your signature, and press copy.  Now go to Settings > Mail and add your signature by pasting it into the signature box.  Your image will be included. If you used different font sizes, they will all be the same now, but Bold, Italics, etc. will be kept.  Even links have the URL intact.  When you send a new mail from the phone, you might not see the image in the mail you are composing, but as the URL is present in the underlying HTML code, the receiving party will see your image in your signature.  This method seems to work with iOS 10.2 PB 3.