Depends on the game.

There’s a lot of good CG art for various sci-fi games and franchises. I used some of the fan-created stuff for the Battlestar Galactica RPG I ran. For the Hollow Earth Expedition campaign, there’s a lot of deiselpunk art, but more likely old planes and car pictures, and the occasional fantasy rendering of places like Shambala. Tales From the Loop comes with its own art that is inspirational. I like to find pictures of faces that fit characters.