Objective-C
//set off-image
mySwitch.offImage = [UIImage imageNamed:@"off_image"];
[mySwitch setOffImage:[UIImage imageNamed:@"off_image"]];
//set on-image
mySwitch.onImage = [UIImage imageNamed:@"on_image"];
[mySwitch setOnImage:[UIImage imageNamed:@"on_image"]];
Swift
//set off-image
mySwitch.offImage = UIImage(named: "off_image")
//set on-image
mySwitch.onImage = UIImage(named: "on_image")