Dear Student,
Photographs are reliable sources of studying history because they picture the conditions as they were, literally. The photographer can not be subjective in the scenes portrayed. They are objective works and depict the situations as they truly existed. Authors and writers present in their works historical incidents and facts as they perceive and sometimes are even contemptuous or prejudiced about accounts they narrate. But this is not the case with photographs which depict the scenes as they were. Hence, photographs are reliable sources of history.
Regards.