在程序中如何把两张图片合成为一张图片

在程序中如何把两张图片合成为一张图片   
- (UIImage *)addImage:(UIImage *)image1 toImage:(UIImage *)image2 {  
    UIGraphicsBeginImageContext(image1.size);  

    // Draw image1  
    [image1 drawInRect:CGRectMake(0, 0, image1.size.width, image1.size.height)];  

    // Draw image2  
    [image2 drawInRect:CGRectMake(0, 0, image2.size.width, image2.size.height)];  

    UIImage *resultingImage = UIGraphicsGetImageFromCurrentImageContext();  

    UIGraphicsEndImageContext();  

    return resultingImage;  
}