To create an HTML-rich Template, go to the Templates library by clicking "Open Templates" in the Outlook ribbon. Then select "New Template" in the upper right. 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, select the "insert/edit image" icon in the formatting options. From there, you can either browse your computer for a specific file, drag and drop an image, or add the source code URL for an image. Then, select "Okay." Finish typing up your Template and save it - you’re good to go!