iPhone: The Contacts app on the iPhone can be linked up to Facebook or your address book to pull images from, but that's it. Contact Pics+ is a simple little app that digs through Twitter, Facebook, Instagram, LinkedIn, Google Image Search and more to find images.
Once you connect Contact Pics+ to the services above, you can browse through them and pick out an image that you like for their contact image. That will then show up in the Contacts app or up above in the recents menu.
It's essentially a one-time use app that you'll only need if you're bent on making sure every contact has an image, but it's helpful nonetheless to fill in the gaps that Facebook leaves.
Contact Pics+ ($2.49) [iTunes App Store]