Yep, it was an enlarged photo. It was a process which became quite popular, and can also be seen on a number of late war 9th AF P-47's.
I'm not sure about Bong's aircraft (might have been varnished in place), but certainly some aircraft had the image fixed in place by a process similar to the dry-mounting system developed by Ademco, where an adhesive tissue is bonded on to the back of the photographic print, in a purpose designed, heated 'press' (it can also be done 'manually', using an electric iron similar to a domestic clothes iron)..
This 'tissue' has a 'double sided' adhesive film, which will then allow the treated print to be 'dry mounted', again by heat and pressure, onto various substrates, a popular example being canvas, or a 'canvas texture' stiff paper, to give a 'canvas' picture which then resembles an oil painting. It was very popular up to the 1970s and 80s, particularly in the 'States, for wedding and portrait photography.
The photographic image can actually be 'stripped' from the photo paper base, leaving the developed photo emulsion on the dry-mounting tissue, to allow the image to then be bonded onto various materials - flat, curved, tubular etc etc - such as wood, ceramics, metal, glass and so on.
As far as I know, the system, known as ADMT - Ademco Dry Mounting Tissue (or Technique, when describing the process) - is still being manufactured and used today.