I have Photoshop Elements and Photoshop and you do it the same as you do letters. If you don't know how to do that, there are others better than me (Michelle and others. But you open the picture you want in the Mickey head and copy it to a new layer. Then put the Mickey head in as a layer and select just the head. I use the magic selector outside it and invert the selection. Then you will either make a layer mask or right click and select layer as copy. The new layer will be a copy of the old layer. Then you can cut and paste it into a new image or delete the old stuff out of the old image, whichever you want.
I hope these instructions make sense. I am not very good at it and leave those designs to others. I keep thinking I should practice more, but I never seem to get around to it.
Good luck and have fun with it.