设置如下布局后，图片和文字都居中显示。

button.titleEdgeInsets = UIEdgeInsetsMake(0, -button.imageView.frame.size.width, 0, 0);

// button.imageEdgeInsets = UIEdgeInsetsMake(0, 0, 0, -button.titleLabel.frame.size.width);

// 由于iOS8中titleLabel的size为0，用上面这样设置有问题，修改一下即可

button.imageEdgeInsets = UIEdgeInsetsMake(0, 0, 0, -button.titleLabel.intrinsicContentSize.width);

如果想图片在上，文字在下，水平居中显示，则按下面设置即可：

button.titleEdgeInsets = UIEdgeInsetsMake(0, -button.imageView.frame.size.width, -button.imageView.frame.size.height, 0);

// button.imageEdgeInsets = UIEdgeInsetsMake(-button.titleLabel.frame.size.height, 0, 0, -button.titleLabel.frame.size.width);

// 由于iOS8中titleLabel的size为0，用上面这样设置有问题，修改一下即可

button.imageEdgeInsets = UIEdgeInsetsMake(-button.titleLabel.intrinsicContentSize.height, 0, 0, -button.titleLabel.intrinsicContentSize.width);

如果觉得图片和文字离的太近了，稍微分开一点：

CGFloat offset = 40.0f;

button.titleEdgeInsets = UIEdgeInsetsMake(0, -button.imageView.frame.size.width, -button.imageView.frame.size.height-offset/2, 0);

// button.imageEdgeInsets = UIEdgeInsetsMake(-button.titleLabel.frame.size.height-offset/2, 0, 0, -button.titleLabel.frame.size.width);

// 由于iOS8中titleLabel的size为0，用上面这样设置有问题，修改一下即可

button.imageEdgeInsets = UIEdgeInsetsMake(-button.titleLabel.intrinsicContentSize.height-offset/2, 0, 0, -button.titleLabel.intrinsicContentSize.width);

文字左对齐，图片右对齐

button.titleEdgeInsets = UIEdgeInsetsMake(0, -button.imageView.frame.size.width - button.frame.size.width + button.titleLabel.intrinsicContentSize.width, 0, 0);

    button.imageEdgeInsets = UIEdgeInsetsMake(0, 0, 0, -button.titleLabel.frame.size.width - button.frame.size.width + button.imageView.frame.size.width);