No, it actually is Stormcloak controlled.
It's http://www.elderscrolls.com/skyrim/media/screenshots/markarth/ btw, in case you were confused with another town.
I'm not confused. Look at the video. There are red flags (Imperial) all over western Skyrim. Markarth has a red flag over it, so it's Imperial-controlled.
http://www.youtube.com/watch?v=fn0N294NFy0&feature=youtu.be&t=25s