There are a few reasons why you might decide to replace the face in one photo with a face from another photo. If you are taking group photos, you might find that no matter how many shots you take, someone is always blinking or making a face. In this case, you can pull a face from another photo to create a perfect shot. Many people also like to replace faces to create joke or novelty photos. For example, you might want to put your cousin standing next to a celebrity. Adobe Photoshop Elements makes it fairly simple to create this effect.
Open the photo you want to pull the face from.
Use the Lasso tool to trace around the face. This selects it.
Go to the "Select" menu and choose "feather". Feather the selection by 5 pixels. This softens the edges of your selection so that it will blend in when you paste it.
Press "Ctrl" and "C" on the keyboard to copy the selection.
Open the photo you want to paste the face into.
Press "Ctrl" and "V" to paste the selection.
Move the face into the proper position.
Use the "Free Transform" tool in the "Image" menu to resize and rotate the face if necessary.
Try to use a face that matches the quality and lighting of the picture you will be pasting into.