For Android Phones---------------
1) Open File Manger or say "my files".
2) Go into Whatsapp Folder
3) There will be several folders such as Backups,Media, Profile Pictures etc. Open Profile Pictures folder.
4) There we can see Contacts Pictures with a one extra file which is " .nomedia " .
5) Delete that .nomedia file.
And thats it. You are Done !!!
Note:- Deleting .nomedia file is totally safe. If in future you want it back then just restart your Phone. :)