So far I've been screenshot'ing my bookshelf and turning them into Folder icons for my archive collection... i started a Wishlist item. So maybe GOG would release the icons they used as "bonus" material :)

does anyone else have a better idea or like this one?

I think for most games, the cover is already included as a hi-res icon.

inside the install directory there is a chance for a fancy icon, i guess i could just go that way, it works too... but im lazy and its easier to take screencaps for now :)

Indeed. I just tried it, and even though there is a hi-res icon of the cover in the .ico file, you can't choose it if you try to change the icon of the folder and browse to the .ico file. There's only one choice (even though there's 9 images in the file), and it's not the cover.

So yes, taking a screenshot, cropping it down and saving it as folder.jpg is probably easier.