Artwork is of significance to the narrative. Without it, the reeemergence of Melinda's voice becomes highly unlikely. One of the most compelling aspects of the entire narrative is the demand that an appropriate response to trauma is to engage in some path that will allow voice to return. Melinda cannot do this through conventional means that high school offers. She cannot be a cheerleader because they are "just too perfect." She cannot really do this through schoolwork because her academic teachers are not very inspiring. Mr. Neck would be an example of this. She is socially ostracized, being blamed for calling the police. Melinda's parents cannot understand one another, and thus do not really understand Melinda.
This leaves art as the realm in which Melinda is able to reclaim her voice. Artwork is important to the narrative because it is the realm in which Melinda is able to find her voice again. Through Mr. Freeman, Melinda is engaged in a realm where she is able to "speak." When Mr. Freeman says to Melinda, "Speak up for yourself—we want to know what you have to say." it marks a moment in which one sees why artwork is so important to Melinda. Artwork is so important because it is the realm in which Melinda is able to reclaim her voice and become a whole human being once again. It is the realm in which voice is activated.
Art is what Melinda used to "speak" out. She would never actually talk about her feelings and what she is going through, but with her art she expressed what she felt without having to speak at all.