The purpose of the image is to guide your gaze to the ad. And nothing does it better than a scantly clad beautiful women. It is like the oldest trick of the trade.

Ever wonder why all those billboards feature beautiful women when the product has nothing to do with women? If you try, you can recognize how your gaze is captured by the image of the women (thanks to a strong evolutionary instinct built over million of years to quickly identify potential mates), and how your gaze naturally follows to the name of the brand/caption on the board...If it was only the brand name/caption, then you would never have even looked at it.