With Steam I usually take screenshots of nice scenes, good action scenes I can use for reference when I draw and so on since some games give some nice ones and have some nice graphics from time to time. Usually for a game I might take about 20 or 30 through the course of it. With Witcher 2, I took 438. =]
These are some, remember some of the lowest settings possible and was at a 1024x780 resolution on Windowed mode so a lot of the textures are worse than they could be and Steam also adds a few pixels when it screens.