I'm no expert on baseball, but if you apply the right-hand rules of pitching to a left-handed pitcher throwing a slider with a more pronounced sidearm style, the batter will see a ball spinning with a pronounced horizontal spin around a vertical axis. The ball will then tend to break away from a left-handed batter and towards a right-handed batter.
The fault in the photo would be that I spun the ball in the wrong direction or that I used a right-handed pitcher. Guess I'll head to the mound for another shot.