There's two ways that you can create an HTML-rich template in Gmail:
1. Create the message exactly as you wish, using the Gmail compose window. Once you've created your content, click on the Templates menu and then click "New Template". That will pull the content of your Gmail compose window into the template editor and allow you to save it as a template.
2. Click the template menu and then "New Template". On the right-side of the formatting tools, there's a "< >" button. Click that to create your template directly from HTML.
To use images in templates, the image must be a publicly hosted link. You will have to host your image on another location and provide that as the "source". You can find detailed instructions for our Gmail tool here. You can find detailed instructions for our Outlook tool here.