It's actually funny how they almost go out of their way to not smile in pictures. You can see a big group of people laughing, having fun, and smiling. When someone tries to capture the moment by taking a picture, the smiles fade and instead, a neutral look appears, and only then is the picture snapped.
The German (or maybe European?) smile-less picture phenomenon is one of the many non-verbal behaviors that differs from Americans. No head nod, no wave, no acknowledgement of of people passing you in the street (unless of course you know them), but most of all, no smiley pictures.