Is this only in photos where a flash was used? I ALWAYS blink when a flash is used. I mean ALWAYS. I hate it when people want to do posed group pictures, because they end up with a lovely picture with this one doofus-looking person (me) with her eyelids at half-mast, looking like some kind of drugged weirdo.
I've learned to not look into the camera, but to look away a bit, and if the person taking the picture insists that everyone look into the camera, I say, "Okay, but I always blink when there's a flash, and my eyes will be half-closed." It's amazing how many people don't believe it. So, I look into the camera, and they end up with a crappy picture, but it's their fault. Still, it would be nice to have a decent photo of myself.
Anyway, I'm guessing that's what's happening with you. If it happens when there is no flash, then, um, maybe you just walk around with your eyes half-shut.