You can assign any picture for any contact, nevertheless of whether they've selected one. This image selection only seems in your Gmail interface whenever you place the mouse over that contact; your other contact won't see it.
Easy Steps for how to assign picture for a Gmail contact:
1. Login Gmail
2. Click Gmail dropdown menu at top left side of Gmail page and select “contacts”
. Choose your contacts from the list that you’d need to add picture, click on their name.
4. Hit “add a picture” in the photo area just next to the contact’s name.
5. Now in the pop-up panel use my computer; Picasa web album; or URL to select the desired picture.
6. Hit “select image”.
7. Crop the picture if liked.
8. Click “set as contact photo”