Being able to rotate (rather than simply flip) a background image would be superb. Implementing this in degrees would be even better. For example:
background-rotate: 0; (default orientation)
background-rotate: 90; (clockwise 1/4 turn)
background-rotate: -45; (anti-clockwise 1/8 turn)
…etc. It would save on resources, and open the door to some new and interesting user feedback too.