I think the 2nd ("game over") and 3rd are night-time pics so they're just dark. The other two are moodily dim but seem bright enough to me.
My brightness is set to 50 & my contrast is 100(max) on my monitor (not vid card) - not that tells you much, since you don't know what monitor I'm using. :D
I have to turn the brightness way up on my monitor to see anything. At the current level (36 Bright/100 Con) I can see:
Pic 1: The gun. Everything else except the sky is completely black.
Pic 2: Same as the first pic. I can see the topmost slash only where it runs across the sky.
Pic 3: Once again I can't see anything except the sky.
Pic 4: I see the tub with the skull, a small patch of greenish wall texture above the crosshair(I think, I can only see a white dot, but if I turn up the brightness the rest of it appears.) and of course I can see the eyes.