You might decide to crop a face from one photo into another for any number of reasons. Sometimes you may be dealing with group photos and just couldn't get a perfect shot of everyone at once. Sometimes you want to do it just for fun. It can be amusing to place a photo of your friend next to a famous celebrity or in the crowd at a well-known event. Using digital photo editing software, you can replace faces fairly easily.
Open the photo from which you want to crop the face in your editing software.
Select the face with the lasso or free selection tool, depending on what software you are using.
Copy the face. Open the photo you want to crop the face into in a new document on your editing software.
Paste the face into the photo onto a new layer and move the face into the proper location. Try to position the new face onto the old as closely as possible.
Resize and rotate the face if necessary. Some editing programs have a free-transform tool that will allow you to do this all at once. Others will require you to resize and rotate separately.
Blur the edges of the face using a feather tool if your software has that option. This is done by selecting the face, choosing the feather option and feathering by 2-10 pixels depending on the size of your photo. Feather more for large photos, less for small photos.
Merge your layers and save the photo as a new file.
If your editing software allows you to work in layers, keep the pasted face on a new layer so you can edit brightness, contrast, etc. to more closely match the photo you pasted into.