I don't think you can. I also tried and no luck. I tried so many different ways that I gave up in the end. What you can do is multiply that picture, I mean put Mickey's signature on one and save it and put Minnies's signature and save the same picture. In the end you will have 3 pictures, 2 of them with signature and the original picture with nothing on it. Hope that made sense.
All the best!