Almost every JPEG or HEIC photo carries an EXIF block. It records the camera or phone model, lens, ISO, shutter speed, aperture, and the timestamp. When location services are on, it also records latitude and longitude down to a few metres.
That's useful for photographers organising libraries, but risky when you post photos publicly — a listing photo of your car can leak your home address. Most social networks strip EXIF; email, Slack, and direct downloads usually don't.
You can view and remove EXIF in the browser without uploading the picture anywhere.