Sure thing. So first you create a central folder for all of your images (in can be anywhere) by default the downloaded images from the scraper reside in /home/pi/.emulationstation/downloaded_images/
But in your case it would probably be easiest to just keep the pictures in the same folder as your ROMS for each system (so just in case you need to back things up they are all in the same place) so for a new nes game your images would be in
Or to keep it simpler you can keep them all in one folder- its up to you.
There are two ways you can add the images to emulation station
You can either add them directly to the gamelist.XML for each system in
You’d change the existing image file path in the line <image> to your new image file path
After you’ve copied your images to your raspberry pi somewhere, while in emulation station with a keyboard you can go into a system like the nes and then for each game instead of opening it you will press select and there will be an option to choose edit this games metadata.
Select that and go down to image and then enter the new file path to your new image.
Then important part- to save changes you’ll need to choose the option from the menu “quit emulationstation” rather than restart system and that will write your changes. Reboot and they should all be there and you’ll be good to go