Here's a nice way to determine if it's legal.
If you're pulling the game screen from the game itself, either through a ROM or a screenshot or some other means, it's *likely* an IP violation. It largely depends on if said game is even properly copyrighted.
It's one thing to *recreate* a screen. It's another to flat out inject it to your game from the original.
Of course I'm no lawyer but the debate can be boiled down to "are you recreating the experience, or are you stealing it from the original source"? The former's a little more acceptable than the latter. Recreating the experience is *your work*. Taking a screenshot is *their work* despite the "work" you did lifting it.