iPhone: How to use CGContextConcatCTM for saving a transformed image properly?

I am making an iPhone application that loads an image from the camera, and then the user can select a second image from the library, move/scale/rotate that second image, and then save the result. I use two UIImageViews in IB as placeholders, and then apply transformations while touching/pinching.

The problem comes when I have to save both images together. I use a rect of the size of the first image and pass it to UIGraphicsBeginImageContext. Then I tried to use CGContextConcatCTM but I can't understand how it works:

CGRect rect = CGRectMake(0, 0, img1.size.width, img1.size.height); // img1 from camera
UIGraphicsBeginImageContext(rect.size); // Start drawing
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextClearRect(ctx, rect); // Clear whole thing
[img1 drawAtPoint:CGPointZero]; // Draw background image at 0,0
CGContextConcatC开发者_如何学JAVATM(ctx, img2.transform); // Apply the transformations of the 2nd image

But what do I need to do next? What information is being held in the img2.transform matrix? The documentation for CGContextConcatCTM doesn't help me that much unfortunately.. Right now I'm trying to solve it by calculating the points and the angle using trigonometry (with the help of this answer), but since the transformation is there, there has to be an easier and more elgant way to do this, right?

Take a look at this excellent answer, you need to create a bitmapped/image context, draw to it, and get the resultant image out. You can then save that. iOS UIImagePickerController result image orientation after upload