Most of the items I sell are personalized, and therefore it wouldn't be appropriate to show an actual photo.
So, keep in mind I'm shooting from the hip based on YOUR scenario - I wonder if you could set up the listing as a 'new item' (under the main setting, list new item). Use a generic starter photo. When the custom painting is complete, edit the listing to add the new photo, and change the order of pics? Or, can you not edit the listing once the listing is sold?
Alternatively, when I create a new design for someone, I almost always will list the design (altering any personal info) once it's sold. In other words, could you take a photo of the custom painting and list it stating in the listing that this is an actual photo of an item that has already sold. There may be slight variations of the newly purchased painting, or something like that?
Good luck. I really stumbled on getting my point across. Maybe too close to bedtime to be offering advice, haha!
(Na-nite!)