If you play Sun and Moon, in the game menu there is the
QR Scanner option.
For citra, you go in Configuration - > Camera -> Rear Right Camera, Select
image camera mode, and put the filepath for the image you want to scan in the textbox below.
The configuration isn't finished yet, in the next build i will make it more user friendly
One more thing, if you want to change the image for the camera, you have to turn off and back on the camera in the game (in Pokemon you can click the Island scanner and go back to QR scanner). There is no Live update for the image right now.