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 - you’re good to go!