by default, user can go : File > Option > Mail > Signatures but there is only plain text!!
user has to copy his premade html (ex test.html) to %appdata%\Microsoft\Signatures
then go again to Outlook > File > Option > Mail > Signatures
source http://myhtmlsignature.com/installing-your-email-signature-in-outlook-2013/
origin - http://www.pipiscrew.com/?p=2187 outlook-2015-html-signature