At the end of the episode, it is heavily implied that the mural Riley painted on the house was Huey and Riley's deceased parents. As evidence from the art teacher was that he asked Riley to paint somebody that he loves who is no longer living.
The original title of this episode was "The Joy of Grafitti".
The art teacher in this episode heavily references Bob Ross, a painter well known for his happy little landscapes and painting tutorial videos.
The comic strip with the talking penguin that Ruckus refers to is "Opus" by Berkeley Breathed. Aaron McGruder is a big fan of Breathed's work.