On an HTC Android phone, the default Contacts/People image is located in the
HtcContacts.apk file located in \System\App folder. (If you copy the APK file to your computer and rename the extension as ZIP, you can then browse the files inside the "zip" file.)
You will find it in
res\drawable-hdpi\people_home_defaultphoto.png
The default incoming call image is located in the
Phone.apk file located in \System\App folder.
You will find it in
res\drawable-hdpi\phone_default.png
The images may vary depending on phone model. These are the pics from an EVO.
As to how you would copy the changed APK files back to the device, I am not sure because I am guessing the Phone.apk and HtcContacts.apk files are running while the device is on — not 100% sure.
Hope this helps!