1. By this statement the authors are simply stating that while slaves were made "freedmen" by Lincoln's efforts and by the victory of the North over the South, in everyday reality they were hardly on anything resembling an equal social, political, legal or economic footing with whites. Whites still occupied the top rail and blacks---freed or not---still occupied the bottom rail.
Histories of the era, say the authors, are inaccurate in part because the blacks studied in those histories were part of the "higher social classes" (172). Therefore, in general, the misimpression is given through these histories that freed slaves were far more liberated and far more able to take socioeconomic advantage of that liberation than in fact most of them actually were.
History is written by those with power, and according to the biases---conscious and unconscious---of those with that power. In this case, whites wrote the history of the freedmen, and it was flattering to those whites to believe that their efforts to free the slaves had paid off. The more socioeconomically empowered freedmen, therefore, were sought out by white historians as the primary sources of the experience of ex-slaves. This fact gives most histories of the era a more positive portrait of the conditions of freedmen's lives than actually existed. For most slaves recently freed, liberation did not mean that those freedmen suddenly moved from the bottom rail of society to anywhere near the top rail. The latter was still primarily the territory of whites and a very few blacks.
2. Holmes' statement simply means that we must keep in mind, when looking at a photograph, that that photograph is not an accurate portrait or reflection of reality. It is inevitably and often deliberately deceptive. The photographer arranges the photograph so that his point of view dominates the photograph and influences the reader to share that point of view. This is as true of photographs of immi...