There are like 750 images in total, that are strictly for the phones GUI and then more for individual apps. there are keyboard images, UI images, Ui Tab bar, Ui button, progress gear, and more and more. I have changed about 250 images so far, and still not all of them show up. There are duplicates with different names that I think are called up at different times. so Although I have changed some images that I think will show up they are only flashing for a brief moment and then reverting back to another stock photo.
Joilange- the image you are looking for are located in /applications/mobilephone.app
you will need to use a program to view the images once you get them. I know there is an app for windows to make the iphone images visible, but im on mac so I use this site
here. Im sure a windows person will be able to help you out with that.
you will see they are called like add_contactpressed.png and add_contact.png and so on so you shouldnt have too much trouble picking out what you are looking for. you might as well edit the entire dialer while your at it.