How to take a screenshot

The photomode mods in general don't come with the ability to take a screenshot of the game. The main reason is that this ability is better left to the tools which are specialized in this.

To take a screenshot of a game, there are a couple of tools available which do a good job. These are listed below. All are free to use. A 'screenshot' feature in a game's photomode is often not desired, as these often take shots in a pre-defined resolution and don't take into account effects you have injected through e.g. ReShade.

This is an effect system which allows you to add effects like Depth of Field, color grading and much more to the game as well as allows you to take a screenshot of the game. Website | Guides @ FRAMED
MSI Afterburner
This is a tweak tool which also offers the feature to take a screenshot of your game. In general it works very well and can capture most games and also the effects you might inject using Reshade. Website
NVidia Geforce Experience
If you have an NVidia card you can use Alt-F1 to take a screenshot of the game, as well as apply effects using Alt-F3.
UPlay / Steam Overlays
These launchers have the ability to take screenshots from games, though it's crucial to configure them so they take uncompressed shots. Often they're configured by default to take compressed JPGs which will ruin your shots. Best to pick BMP or PNG instead.
XBox Game Bar
Windows 10 comes with the XBox app which offers a 'game bar'. You can open this gamebar with windows key + G. To take a screenshot of the actual game window, press windows key + Alt + PrintScreen.