Download the cat and dog head and 2 of the portraits.
Open the photo of the person first in photoshop
File>Place embedded the cat or dog photo
Make it larger than the persons head.
Click remove background for the cat or dog photo
Use your BRUSH TOOL (B key) with the color BLACK IN FRONT to mask out parts of the cat or dog head
Click the X key to swap the Black and White color chips
Black makes it disappear, White makes it come back
Use the MOVE tool to move the head around.