I've experienced this as well with my dad's photo years and years ago. Thought it was my eyes playing tricks on me but it's not happened another single time. And I've stared at that picture quite a lot.
Have always believed he was letting me know he's still here with me.
Funny things happen with pictures around here what with seeing my dad's face begin to animate in his picture and my mom's picture falling from the shelf 3 times for no apparent reason.
