Reshade uses the Printscreen button for saving screenshots in the same folder where the game exacutable is. It saves in png format by default, but you can change that to .bmp by editing the configuration files.
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With ReShade 0.19.2, it will not. But you can use other softwares that will capture ReShade's effects to take the screenshot and replace the ones took with Steam. Ain't so hard, really.
I use Bandicam to do that, I move the ones I like more to the game's screenshot folder, rename them to the Steam's naming way and then I upload them.