Taking my current profile picture as a prominent example:
This photo has been taken with flash. As I gather, this is the same effect that makes a human's eyes glow red on photos, namely the reflection of the flash at the choroid and then passing back through the retina.
The retina is full of bloodvessels and the blood inside them gives the reflected light it's reddish color.
If it's the same effect, why is the color different?