I know that your art history professors want you to believe that art was used to create social change. Maybe in some instances it wasn’t the spark that created the social change but helped it along. For example, when Jacques Louis David painted his Oath of the Horatii, it was at first thought of very kindly and accepted by the aristocracy and by the King himself. However, David’s painting is taught as a sort of icon of political change. I think that historians either through lack of facts or by a forcing a thesis have taught that David’s paintings were the cause of change rather than being a document that expressed.
Another instance where it’s been taught that fine art created social and political change is in the work of Honoré Daumier. In fact I think you might even been jailed at one point for making some cartoons of the Emperor. He certainly was censored. However, I think that David and Daumier were both recording and expressing ideas that had already been floating around at the time.
FlashForward to the 20th century and think about the WPA artists such as Thomas Hart Benton and Dorothea Lange. Art historians are very fond of saying that they were causing social change but if you think about it they not so much caused it as recorded it.
Today, we have artists like Banksy, who do comment on society and have even gone to the Middle East and created art installations but I don’t think that they’re changing anybody’s minds.
The bottom line, and I guess my main thesis in answering your question, is that art is often created as an expression of ideas that people already agree with. The art that is sold and even studied in art history classes are often taken up for political reasons by the people who support the causes. I don’t really think that they create the social change as much as express it. Of course you can probably fine art historians to disagree with me and that’s fine I just can’t think of a single example where work of art actually created social change, even in the posters that were in favor of Obama.