Quote - Most images (even those taken from a camera) are already gamma-corrected. To use those images in a render (as background or texture) the gamma effect has to be reversed

Cameras have not gamma correction, and are made to have a linear response. TV stations correct the gamma at other stage and not at the camera.
I don't know about digital TV, the standards are different.

Photographic images also also not linear and much is corrected in the laboratory.
The non linearity of a photograph is not the same as a cathode ray, plasma tubes has other non linearities.
Photographic images are not made to be seen on monitors, are made to be printed, so no cathode ray tube is used and the gamma corrections are very different.

The famous nice curve x^2.2 is only for the cathode ray tube at voltages near the grid cut voltage (black level). For other intensities the curve is more or less linear and when the grid saturates it begins again very non linear (ultra white).
If you set the intensity of your monitor for strong daylight illumination and the change the intensity to work at night with little ambient illumination the gamma correction required in both cases are different, so you will need to render a scene to watch at day and another to watch at night.
Of course that you can add gamma correction to make your scene look nicer and Photoshop has a lot more plugins to make your image nicer too.

Stupidity also evolves!