PHP
(quick guide & examples)
MAIL MESSAGE $mailBody= "<img src='http://site.com/img.jpg'>";
MAIL HEADER $mailHead = implode("\r\n", [ "MIME-Version: 1.0", "Content-type: text/html;". "charset=utf-8" ]);
SEND MAIL mail("jon@doe.com", "Subject", $mailBody, $mailHead);
MAIL MESSAGE $img = file_get_contents('IMG.JPG'); $imgdata = base64_encode($img); $mailBody = "<img src='data:image/x-icon;base64,$imgdata'>";
MAIL HEADER $mailHead = implode("\r\n", [ "MIME-Version: 1.0", "Content-type: text/html;". "charset=utf-8" ]);
SEND MAIL mail("jon@doe.com", "Subject", $mailBody, $mailHead);