In Settings - Facebook, enable Contacts. That done, select "Update All Contacts" below:
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In the Contacts app, tap "Groups" at the top left. Make sure Facebook is enabled. If it is, drag downwards the list of accounts to force them to sync to your iPhone. A spinning wheel will appear at the top.
Select "Done" afterwards, and look at your contacts now. Usually they should show the FB images then.
Hope that helps.