How does One place an image inside of an image using metadata?

For example, I can convert the second image into NSData and then place it inside metadata inside the first image and then when I open the first image and read the metadata I can get the NSData and turn it into an UIImage.

How would I go about doing this? All the metadata tags I see are not large enough to support another picture. I know picture in picture is quite common on deskt开发者_C百科op apps so I'm interested in getting it to work on the iPhone.

Is metadata the correct way to do this or is there another way?