Hellu thread bears and papagator!
Papa you wondered about the paint somehow effecting the quality of your guitars sound.    I a have a friend who painted the entire soundboard of his guitar, personally I preferred the look of it before but there was no discernable difference in tonal quality.   The guy who made my mando, I think uses shellac because of some sort of relationship it has with the wood and its resonance(I think it was something like this). It All sounds a bit sordid but he obviously thinks there's something in it and his instruments do sound divine. No matter what you do with your soundboard there will be a difference in it's response(often this will not be mesaurable by human ear), but you really have to do something way more drastic than painting a little picture, to make a difference that your ears will percieve, and anyway the plastic pickguard you put over the picture will effect soundboard movement more than the pic itself and that's not very much at all!. muhkuhpuh