In order to understand the significance of the contrast between the two pictures, it's important to get an overview of the entire story.
"Soldier's Home," by Ernest Hemingway, tells of a soldier named Harold Krebs who comes home late from World War I to a town in Oklahoma. The townspeople no longer want to hear about the war, and, after a while, he no longer wants to talk about it. He becomes listless, sleeps late, and wanders around without knowing what to do with himself. He enjoys looking at the pretty girls but lacks the energy or courage to pursue them. In his mind, he contrasts the girls in his home town, who are prettier but harder to get, to the French or German girls who were not as attractive but easier to befriend.
When Krebs sits on the porch and wishes that the book he is reading had more maps, it is obviously a reference to the fact that he has no direction in his life. When Harold tells his mother that he doesn't love her and he's not part of God's kingdom, it's an indication of his dislocation. In Hemingway's spare style, he uses these details to show how much the experience in the war has affected Krebs.
In the first two paragraphs, Hemingway gives the description of the two pictures as an overview of the entire story. In the first picture, Krebs is in college with his fraternity brothers, and they are all wearing exactly the same collars. This shows that Krebs fit perfectly into society—just like everyone else—before he left for the war.
In the second picture, Krebs is in Germany; his clothes don't fit properly, and the girls aren't very pretty. This highlights his dislocation. He is in a foreign country—far from home and away from the society in which he was raised. When he comes back, he can't fit in anymore.