Thanks for getting back to me. I seem to have misunderstood your initial query a bit, but I understand you more correctly now (apologies for that). Yes, your understanding is correct - the templates do have defined image sizes, so the supersized version of images would show at those dimensions (1390 x 1040) when you have used the larger of the available sizes for upload.
There is another option, which may or may not be suitable for your purposes. You can also use the Insert an Auctiva Hosted image tool in the description editor to add your uploaded images directly to your listings at the larger size. This will display such images at that size within the description - so they would not supersize and the listing would have large images within the description. While this would not resolve any issues with the Header image as that must be submit to eBay using the Image Selection area of your listings, it would allow you to have images at 1920 X 1440px within the listing itself.
The tool for this is located in the lower portion of the toolbar for the description editor. It is the button just beneath the button for Bold (9th from the left).
Should you encounter any difficulty using this tool or have any other difficulty, please also feel free to file a support case from under the Help tab of the site so our support team can have a look directly into your account at some examples to assist you further.