About Offline Preview

So i finally managed to get a little bit better understanding of using mediator lately, and here's a question about offline preview,
i found out i can still apply various effect onto offline preview without game running,
but then i can no longer switching enable and disable reshade , coz everytime i turn off and on any effect the picture for preview change at once,
so i got no idea how the original picture should be without disable and enable for comparing,
guess i'm doing something wrong, so anyone have any idea how this happens?

guess i was wrong, the game has to be running for mediator to work,
but another problem is if i took a screenshot and edit with ps, after that using offline preview to open it,with game running background of coz
but still i can't generate LUT with it, L+print screen, nothing,,