Okay, I don't understand why everyone scowled/frowned/pouted/looked p****ed off in old pictures like that! Why did they do that? Was it to look dignified? Although that sounds really dumb considering many pics of old fashioned people are candid (especially the IF).
Michelle,
In early photographs, say prior to the 1890's, there was apparently a long exposure time, so pictures took a longer time to take. Therefore, most people couldn't bother to hold their smiles for as long as it took and it was just easier to remain serious. I don't know much about photography, but this is a common explanation given to this question.
With regards to candid and private photographs, I think that the majority of the pictures of the IF show them smiling, with the exception of Alexandra, whose smile was rare. If they happened not to be smiling, I can only guess that they just didn't feel like smiling because they were either in no mood to be photographed or wanted to be captured in a serious mood. Aside from the IF, other people perhaps just didn't like their own smiles and thought they looked more attractive for the camera with a serious face.
On official photographs, it was always not considered "dignified" to smile. That was just the attitude of the day, strange as it may seem nowadays, when people are encouraged to smile in order to look friendly.