Hi All,
One way to do what you are asking is to create a listing but only fill in the title, description and choose the images that you want to be displayed inside the listing (do not choose your header or gallery image at this time!).
Save that listing then go to the saved listing page, click the radio button to the right of that listing and then locate the "Get HTML" button at the top or bottom of that page. Proceed to copy the HTML.
Then you will want to create a new listing, filling in the top section of the page until you get to the "item description" editor. On the editor you will switch views to "Text Mode" paste in your HTML and edit in the appropriate background image to the main table. From there you will NOT select any more images to place into your template because they are already in the code you pasted into the editor but you may want to choose a "Header Image" and/or a "Gallery Image"
Continue to fill out the remainder of the lister page. Save and then preview.
There are limitations to creating a listing this way but feel free to explore, this should be enough to get you started.