There are two ways that you can create an HTML-rich Template in Gmail:

1. Open a compose window in Gmail and create the message exactly as you want it to appear. Once you've created your content, click on the Templates button at the bottom of the compose window and then click "Save as 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, simply 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!