My experience is that the unlit screens look more washed out than the lit which is fairly intuitive as the lit have a light source behind them so they suffer less from the glare - I suspect that is what is making the difference and in actuality the image is being treated the same by the game (but I might be wrong). I think the PNG, JPEG etc. thing is all a red herring and people are getting different experiences based on the programmes they are using and how they save colour profile data. I posted my explanation of how I got my images to work in gimp here. Using the automatically loaded colour profile didn't work for me despite it saying that it was the right format - I literally had to find an image that did work in game with the right colours and use it to create a custom colour profile from it because none of the fixes (e.g. loading it in certain programmes etc.) was working for the images I wanted. I suspect it is a complex combination of the programmes settings 'deciding' what the correct colour profile for the image is plus the meta data stored in the original image. So some images you won't have issues with and others you will.
As @Danny_zoo said it is a limitation - my own suspicion is that to change it might not be in Frontier's hands without really limiting the types of images and formats that can be used because it is embedded in the image meta-data and Planet Zoo is a game not a bit of software designed to interpret and change images.
If it is at all helpful the image that I found that worked as a fully colourful, bright and as intended picture and that I used as my colour profile template to impose on my images was one kindly made freely available by a forum member. It was the truly stunning clouded leopard by @Ulysse you can find on the thread here
As @Danny_zoo said it is a limitation - my own suspicion is that to change it might not be in Frontier's hands without really limiting the types of images and formats that can be used because it is embedded in the image meta-data and Planet Zoo is a game not a bit of software designed to interpret and change images.
If it is at all helpful the image that I found that worked as a fully colourful, bright and as intended picture and that I used as my colour profile template to impose on my images was one kindly made freely available by a forum member. It was the truly stunning clouded leopard by @Ulysse you can find on the thread here