Photo Mode is currently in Beta and only available in Mission Select after you have completed a mission in the Campaign mode.

You must first enable Photo Mode from the settings menu. Once Photo Mode has been enabled you may press Right-Alt on a keyboard or Down on the D-Pad of a controller to access Photo Mode during gameplay.

Note: Any actions configured to Right-Alt or Down on the D-Pad will not function if you have Photo Mode enabled.

 

 Photo Mode images save here by default: C:\Users\<UserName>\Saved Games\id Software\DOOMEternal\base\screenshots