Where are screenshots saved in EU4?
There are two kinds of screenshot in this game.
You can either take the usual Steam screenshots with F12. This is explained here.
It is also possible to take a screenshot of the whole map with F10. In this case, the file will be located in Documents\Paradox Interactive\Europa Universalis IV\Screenshots.
On linux the folder is ~/.local/share/Paradox Interactive/Europa Universalis IV/Screenshots