Based on the description of the piece of art being a "picture of a black motorcycle," the most obvious purpose of this piece of art seems to be:
an object to express personal feelings or thoughts.
This interpretation could be based on the idea that a motorcycle often symbolizes personal freedom, adventure, or a lifestyle, which can convey deeper meanings or emotions. However, if the focus of the analysis is purely on aesthetics, you might also consider "an object created for aesthetics" as a viable response. But given the context, the first option may resonate more with the idea of expressing personal sentiment.