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!