Posters, or the projection screen?
If posters, archicad has a picture object
If a projection screen, it gets more complex. You need glare and light into the theatre. I have made an article in my book about this topic. To summarize it:
-make a thin wall as your projection screen.
-make a texture map the exact size of the projection screen as a material.
- in photoshop, lighten the texture map toward overexposure, simulating glare.
- apply the material to the wall and use the 3D texture alignment tool to justify the tiling.
- have a light cone light source shine straight out of the face of the screen to illuminate the seating.
Dwight Atkinson