Hi a_right_dodgy_geezer,
If your template is stored within your Auctiva account as a "custom template", you should be able to accomplish this by choosing to edit the template and then clicking the "Standard" tab along the top of the template. You could then add your image by placing your cursor in the desired location and clicking the "Insert an Auctiva Hosted Image" button on the editor toolbar.
This button has an icon of a mountain and sun on it and is located just to the left of the "User Links" drop-down menu.
If you have any additional questions or concerns, please feel free to contact our customer support team using the web form on the following page of our site:
http://www.auctiva.com/help/requesthelp.aspx-Mike