Stuck at 18. The last two are probably quite obvious too.

found 18 so far.

Upto 19. Examined everything again twice more to get that extra one. As to the last one? Reindeer and snowmen in a few shots, brings me upto 23. Ahhh, nah, can't be right.
Good luck to the rest of you.

I think I found 20. Or I missed one, and added one that's ....hard to differentiate the pixels in....
NO CLUES!!

15/20. No luck with the rest.
:(

I think I'm too lazy for this one.

This is harder than I thought. The most annoying are those that seems to be modified but you can't know for sure.

Yes, especially if you haven't played the game in question.
I've found a couple of them, and am content with them. Still, I'm not going to participate, only enjoy finding them if I (accidentally) stumble upon one or two.

Great Idea!

Question to those who found all 20: Were all of them the fairly blatant trees/hat+beard etc. or were some (or 1 seeing as that's what I'm stuck on) vastly different than the rest?

They're varying degrees of spottyableness. And if the one I think may/may not be correct - it's bloody hard - you really have to look twice, thrice and more but it does look like a little santa.
Some you can spot on just the thumbnails, some you have to actually get the larger picture up.

Hmm, I'll try again, as my first attempt seems to have been swallowed up, somewhere...
They've varying degrees of difficulty in spotting. Some are very easy - you can spot them through just the thumbnails on the game page.
Some, you have to click and expand the thumbnails, and click through the larger pictures.
One was, if it actually is correct, and not me seeing something that isn't there, is very, very, very easy to miss - but is a very small santa...

I found all of them to be fairly obvious. If you are not sure about one, keep checking other screenshots and you will see the same modification in at least one other game. There was one game where I was thinking, "is that part of the original game or has it been added?" and I wasn't sure until I saw the exact same image in another game's screenshot.

I went through every screenshot and I found 18!
EIGHTEEN!
Gaaaaaaaah!
I'm tearing my hair out trying to find the last 2 (though I did find a couple of games I want to buy).

I went through every single screenshot of every game and I found 17 or 18, there's a game I'm really not shure.
There's a game where it's damn hard to say if it's part of the game or if it's photoshopped. Can anyone confirm if the find there is correct?
Edit: Ok, I'm shure now, it's correct.
So I got 18 of 20.

I've found 19 screenshots. Maybe I've found the 20th too. I can't understand if the GOG logo shown on a plane (it's on a game screenshot) is part of the contest or not. I'll look all the game pages again, though.