Pip-Boy: Thank you GOG for this contest and congratulations to all the winners!
After I found only 16 screenshots I lost a little bit of my eagerness. I looked then at the URLs of the modified screenshots and discovered a pattern which I think comes from GOG's upload mechanism for content. One could use it for a
trick:
All modified screenshots have a "2009/12" (contest start) in their URL, so instead of looking at every game screenshot one could look for this pattern on every game card, especially in the page info window of Firefox (see attached image e.g. for one from Battle Chess, a very difficult one)
;)
damn i should have figured that out. obviously they uploaded the edited christmas pictures right before the contest started so they all have that unique date. I need to think more like Pip-Boy...