Remove that lowercase "m" in https://i.stack.imgur.com/XXXXXm.png (More Info)
The screenshot has nothing to do with your cameras. A screenshot is taken by the system (or the graphics driver), who takes one frame from its rendering buffer and saves the frame. Thus a screenshot always has the same resolution as your screen, and is always a PNG.
Changing brightness should have no effect on your screenshots taken, because it does nothing to the images rendered by your graphics processor. For LCD screens, brightness is simply the power supplied to the backlight panel, and the masking (front) panel is not affected. To confirm this, find a static screen and take two shots with different brightness levels. Compare those screenshots pixel-by-pixel and you'll find them exactly the same.
For grainy textures, it's Imgur's service for resizing photos that caused that problem.
I've edited your example post and here's the change:
[![enter image description here][1m]] : https://i.stack.imgur.com/L21J7.png [1m]: https://i.stack.imgur.com/L21J7m.png
See for yourself