There are probably many ways, but I use raspi2png (look for it on github)
1) Start RetroPie / EmulationStation on its own screen. And get to the screen you want to ehh, shoot.
2) use ssh to get to a terminal
3) go to the directory where raspi2png was installed (my case: home/raspi2png)
4) run ./raspi2png -p usefullfilename.png
5) use winscp to copy the bew file over to pc
Send from mobile, please excuse any brevity and typos.