You can submit missing images to thegamesdb so others can benefit if the gameadb doesn’t have an image.

You can also transfer over your image and then press shift (or select, I can’t remember) once you’ve navigated to the game and edit the image path manually to where you transferred your image over. Just note that you’ll have to select quit from the emulationstation start menu If you want those manual changes saved.

Otherwise you can manually edit the gamelist.xml for that system which accomplished the same thing.