Well, in black plague if you get all ten artefacts you get the code
Spoiler below!
xor451 which gives you access to wallpapers.
Or you can go to c drive / program files / penumbra / Episode 1/ Redist / (from here i forget) intro and you'll have all the artwork for the introduction of overture.
The tech demo is the sam process, except you don't go into episode 1.
Black plague is listed under Paradox Interactive. hope that helps. =]