To avoid degrading image quality, use UIGraphicsBeginImageContextWithOptions.
If you specify a value of 0.0, the scale factor is set to the scale
factor of the device’s main screen.
UIImage *image1 = [UIImage imageNamed:@"image1.png"];
UIImage *image2 = [UIImage imageNamed:@"image2.png"];
CGSize size = CGSizeMake(image1.size.width, image1.size.height + image2.size.height);
UIGraphicsBeginImageContextWithOptions(size, false, 0.0) // Use this call
[image1 drawInRect:CGRectMake(0,0,size.width, image1.size.height)];
[image2 drawInRect:CGRectMake(0,image1.size.height,size.width, image2.size.height)];
UIImage *finalImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
//Add image to view
UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, finalImage.size.width, finalImage.size.height)];
imageView.image = finalImage;
[self.view addSubview:imageView];