Your photo files actually contain hidden information. EXIF stands for Exchangeable Image File Format, and is a standard for storing interchange information in image files, especially those using JPEG compression. Most digital cameras now use the EXIF format to ecord of the settings a camera used to take a photo or video. This information is embedded into the files the camera saves.
Most of it is pretty innocuous but not all. EXIF data includes the date and time the photo was taken. The model and serial number of the camera are recorded. And depending on the camera, the settings you used may also be stored.