There isn't necessarily an easy answer to that. The simplest way would be trial and error with a photo editing program like GIMP or photoshop. Paste the face on top and then move it around until you get it right. Generally wouldn't be worth the effort unless you want to use them for a Renpy project and need stuff that fits a specific art style.
It's a lot easier when the face images are the same height and width as the body images and are already in the right position.