why sherman alexie decided to omit illustrations when depicting some parts of juniors life

One of the main reasons Sherman Alexie seems to include images is to illustrate some aspect of Junior's life that he wants to make light of, regardless of how serious the issue. The racism he faces, starvation, and Native American social expectations are all game for Alexie, and Junior's artwork, which actually, in a way, mocks these major issues.
However, there are such serious issues in Junior's life that Alexie shies away from making light of with the cartoonish drawings. One such example of this is in the second chapter, "Why Chicken Means So Much to Me." While Junior can make light of how fried chicken from KFC for a "poor-ass reservation kid living with his poor-ass family" can make "anybody believe in the existence of God" with a drawing with the caption "the Shroud of Kentucky Fried," the killing of Junior's dog did not warrant a drawing. Instead, Junior just explains, without image, how being poor meant he was unable to take his dog Oscar, whom he calls his best friend, to the vet and how his father took the pet out back to shoot it. This episode in the novel is poignant because Alexie refrains from making light of it, explaining how "poverty doesn't give you strength or teach you lessons about perseverance. No, poverty only teaches you how to be poor."