Why do people someimes have red eyes is pictures?

The "red " in the eyes of some people is from a camera with a FLASH.

In the flash the person is in a dark setting so this makes their iris dilate letting in light from the flash.

In the back of the human eye is a blood rich retina that turns focused light that hits it and turns the light into electrical signals that travels through optic nerve to the brain. The brain then turns these electrical signals into a arrangement (digital) that our brain understands as a image.

I digress; the "red" eyes in the photo is due to the flash hitting the retina and is reflected back to the lens of the camera