Symbolism, Imagery, Allegory
Here's a Sky is Everywhere pro tip: Almost all drawings and paintings in the book have to do with Paige, Lennie and Bailey's absent mother. It's no secret, really—most of the art is called out in the text as being mom-oriented. Take "The Half-Mom," one of Gram's paintings, for example:
Before she left sixteen years ago, Gram had been painting a portrait of her, which she never got to finish but put up anyway. It hovers over the mantel in the living room, half a mother, with long green hair pooling like water around an incomplete face. (3.51)
Paige just up and left, leaving her life with her family unfinished and incomplete—her daughters don't really know her—and all of this is reflected in Gram's painting. Additionally, in bouncing, Paige turned Gram into the girls' mom; so in only being half a mom herself, Paige also turned Gram into a half-mom, half-grandma hybrid.
Interestingly, Gram's other paintings are similar, and all green. We're told:
She has every hue from lime to forest and uses them to primarily paint one thing: willowy women who look half-human, half Martian. "They're my ladies," she'd tell Bails and me. "Halfway between here and there." (3.35)
So all the paintings are otherworldly and incomplete—just like Lennie's knowledge of her mother, and just like Gram's relationship with her daughter. Until Lennie finds Gram's unsent letters, all she knows about her mom is tall-tale story of her as an explorer, a half-truth slapped together to explain her disappearance from Lennie and Bailey's lives.
Lennie also finds Bailey's old drawing of their mother atop a mountain, labeled "EXPLORER." Lennie starts talking to the drawing, but not as if the drawing is her mother—she uses it as a stand-in for Bailey. Perhaps this is because she sees Bailey as legendary, like her mother, or because she's always thought Bailey had the "restless gene," too. One thing's for sure: Like Paige, Bailey is no longer an active part of Lennie's life.
Toward the end of the novel, we find out more about Paige—details from Gram's letters and confession—but even so, Lennie's picture of her mother is never going to be complete. The book doesn't end with Lennie finding her mom; it ends with her reading the letters to learn more about her, and counting her blessings that Gram wound up being the person who raised her. She may have a fondness for strange paintings, but at least Gram is someone Lennie can really get to know.