We trained Arsenal's machine learning algorithms using a large database of photos from several popular photo sharing sites. (The images are publicly-available and we pay a licensing fee for our usage.)
Arsenal doesn't store any of the training photos locally on your device. Instead, we store a vector -- a mathematical summary of the photo -- that was calculated during the initial training process. This lets us find similar photos to the scene you are shooting and calculate appropriate camera settings without needing to store any training photos on your device.